18世纪末的工业革命，开启了人类社会的城市化进程。随着现代交通的大发展和信息时代的到来，城市之间的地域限制被打破，城市之间联系更加活跃而密切。城市群成为我国当前城市发展新的方向。市场经济背景下，城市发展越来越需要加强与外界联系，提高自身的外向度，同时要保持合理的发展水平。2014年3月，李克强在政府工作报告中，提出京津冀一体化；2015年4月，通过京津冀协同发展规划纲要。京津冀城市群成为我国经济发展的重要区域。城市外向度反映了城市的发展动力，它和发展水平的耦合协调关系到城市的可持续发展。本文以京津冀城市群为研究对象，选取城市外向度和发展水平两个指标体系，分析城市群内两者的总体差异、等级体系，以及两者耦合协调的结构关系，探究作为网络节点的城市，在城市网络空间结构体系中其自身结构的稳定性。研究结果表明：

（1）京津冀地区城市外向度和发展水平存在较大差异，北京、天津远高于河北城市，河北内部也有较大差异，研究期间内没有缩小趋势。京津冀一体化和京津冀协同发展的政策在城市外向度和发展水平方面没有发挥明显的作用。京津冀城市之间差异显著，资源分配不均，不利于区域的协同发展。

（2）京津冀地区城市外向度、发展水平和二者的耦合协调度呈明显的等级体系。综合三者的等级体系来看，北京、天津为一级，石家庄、唐山为二级，保定、廊坊、秦皇岛、邯郸、沧州为三级，张家口、承德、邢台、衡水为四级。这种等级体系与地理空间上的经济分异之间存在响应机制，可以看出，北京、天津的经济外溢效应能够惠及到与其临界的保定、廊坊、沧州；而张家口、承德虽然地缘上邻接北京，但由于自然条件和生态保护等原因的制约，发展受到了限制。石家庄作为河北省会城市，其辐射带动能力不足；与其邻接的邢台、衡水等级一直较低。

（3）京津冀地区城市外向度和发展水平的耦合度除2014年衡水外，均处于颉颃阶段，外向度和发展水平的交互作用程度较弱。且各城市之间的差异明显，除北京、天津较为稳定外，其他城市都有不同程度的起伏。城市外向度和发展水平互为因果，二者结构合理则会相互促进，推动城市可持续发展。而结果显示，只有北京、天津外向度和发展水平的耦合度接近磨合阶段，其余城市均没有呈现接近磨合阶段的趋势。这表明京津冀地区城市外向度和发展水平的交互作用较弱，没有达到相互促进的状态，不利于城市的可持续发展，应注重城市外向度和发展水平的协调发展。

（4）京津冀地区城市外向度滞后于发展水平的现象严重，除北京、天津处于稳定的协调状态外，其余城市均存在外向度滞后或严重滞后于发展水平的问题。这说明，这些城市推进城市化时，忽视了相应提高城市的外向度，出现了外向度滞后的问题。再加上人流、物流、资金流、信息流流向北京、天津，在要素流动的过程中，其余城市缺乏竞争力，最终导致这些城市外向度滞后于发展水平。不管是外向度滞后，还是外向度集中于京津，这两点既不利于京津冀一体化发展，也不符合协同发展的要求。区域的协同发展，不仅需要核心城市的辐射带动，还需要周围城市提供广阔的腹地，河北城市需要注重城市外向度的提高，支撑京津冀协同发展。

The industrial revolution at the end of the 18th century started the urbanization process of human society. With the development of modern transportation and the arrival of information age, the regional restrictions between cities have been broken, and the relationship between cities has become more active and close. Urban agglomerations have become the direction of urban development in China in the new era. Under the environment of market economy, the development of cities needs to contact with the outside world more and more, improve their extroversion, and maintain a reasonable level of development at the same time. This paper takes Beijing Tianjin Hebei Urban Agglomeration as the research object, starting from the urban node, selects two index systems of urban extroversion and development level. This paper analyzes the overall difference, spatial structure, hierarchical system, and the rationality of the coupling and coordination between the two in the urban agglomeration, and explores whether the structure of the city as a network node is stable in the urban network spatial structure. The results show that:

(1) There are great differences in the degree of extroversion and the level of development between Beijing, Tianjin and Hebei Area. Beijing and Tianjin are far higher than Hebei, and there are also great differences within Hebei. There is no narrowing trend during the study period. The policy of Beijing-Tianjin-Hebei integration and Beijing-Tianjin-Hebei coordinated development has not played an obvious role in the city's export-oriented degree and development level. There are significant differences between Beijing, Tianjin and Hebei area cities, uneven distribution of resources, which is not conducive to the coordinated development of the region.

(2) In Beijing Tianjin and Hebei area, the degree of city's outward orientation, development level and their coupling and coordination are obviously hierarchical. According to the three levels, Beijing and Tianjin are the first, Shijiazhuang and Tangshan are the second, Baoding, Langfang, Qinhuangdao, Handan and Cangzhou are the third, Zhangjiakou, Chengde, Xingtai and Hengshui are the fourth. There is a response mechanism between this hierarchical system and the economic differentiation in geographical space. It can be seen that the economic spillover effects of Beijing and Tianjin can benefit Baoding, Langfang and Cangzhou, which are critical to them. While Zhangjiakou and Chengde are geographically adjacent to Beijing, their development is limited due to the constraints of natural conditions and ecological protection. As the capital city of Hebei Province, Shijiazhuang's radiation driving capacity is insufficient, and its neighboring Xingtai and Hengshui have been at a low level.

(3) In addition to Hengshui in 2014, the coupling degree of urban extroversion and development level in Beijing Tianjin and Hebei region is in the antagonistic stage, and the interaction degree of extroversion and development level is weak. And the differences between the cities are obvious, except Beijing and Tianjin are relatively stable, other cities have varying degrees of ups and downs. The city's extroversion and development level are mutually causal, and their reasonable structure will promote each other and promote the sustainable development of the city. The results show that only the coupling degree of Beijing and Tianjin is close to the running in stage, and the other cities have no trend to the running in stage. This shows that the interaction between the city's extroversion and development level in Beijing Tianjin and Hebei region is weak, which is not conducive to the sustainable development of the city. We should pay attention to the coordinated development of the city's extroversion and development level.

(4) In Beijing Tianjin and Hebei area, the phenomenon of the city's extroversion lagging behind the development level is serious. Except that Beijing and Tianjin are in a stable state of coordination, other cities have the problem of the extroversion lagging behind or seriously lagging behind the development level. It shows that when these cities promote urbanization, they neglect to improve the city's extroversion, and the problem of backward extroversion appears. In addition, the flow of people, logistics, capital and information flows to Beijing and Tianjin. In the process of factor flow, the other cities are lack of competitiveness, which ultimately leads to the lagging of the export-oriented degree of these cities. Whether the degree of export lags behind or is concentrated in Beijing and Tianjin, it is not conducive to the integrated development of Beijing, Tianjin and Hebei area, which does not meet the requirements of coordinated development. The coordinated development of the region needs not only the radiation of the core cities, but also the vast hinterland provided by the surrounding cities. Hebei cities need to pay attention to the improvement of the city's export-oriented degree to support the coordinated development of Beijing, Tianjin and Hebei area.