细分的贸易数据显示，一国不同产品的出口额的分布基本一致，少数产品的出口额占国家出口总额绝对份额，而多数产品的出口额占比很低。若对不同产品出口额从大到小排序绘制多度分布曲线，曲线的走势基本一致，DGBD(the Discrete version of Generalized Beta Distribution)[20]公式对其拟合也能得到很高的拟合优度。不同的国家，拟合参数与出口种类数存在显著的相关关系，显示出口种类数少的国家的多度分布曲线比出口种类数少的国家平缓，表明缺乏多样性的国家出口额的分布相对均衡。同时，贸易数据还显示，国家出口的商品种类数与国家的log GDP之间存在稳定的“S”型关系[5]，说明出口种类数的对log GDP的增速先快后慢，最终不再增长。本文引入生态中性理论模型的建模思想，建立了贸易中性理论模型，模型假定一国的贸易额短期固定，通过三种途径（生产已有产品、引进他国产品、生产新产品）实现贸易额在不同产品之间的分配。模型从区域经济系统、全局经济系统和国家经济系统近似模拟全球的贸易网络。结果显示，模拟的数据分布特征与实证数据非常相近，本文为解释出口产品额的分布以及出口种类数与国家log GDP的“S”型关系的形成提供了可能的途径。

The detailed trade data shows that the distribution of the export value of different products in countries are similar. Of all the trade for one country, the shares of few products are very large while those of most products are small. I arrange the export value of different products for different countries largest to the smallest and find the curves are similar and the DGBD (the Discrete version of Generalized Beta Distribution) function can fits the curves successfully. The results show that the estimated parameters in DGBD function and the number of species of export have strong dependency relation, which shows that the abundance distribution curves of countries with low diversity are smoother than those with high diversity. Additionally, the data shows that the kinds of export goods and the logarithmic GDP of a country have an S-shaped relationship. This indicates that the growth of the kinds of export goods to the logarithmic GDP increase first and slow down after. This paper is inspired by the neutral theory model of biodiversity and biogeography. The paper makes up a neutral theory model for explaining the formation of unified abundance distribution of export value and the S-shaped relationship between the export diversity and log GDP. The model assumes that the total output of a country is fixed in a short time, and one country can redistribute her investments among different products randomly through three methods. In the model, local economy system、meta economy system and national economy system are introduce to simulate international trade network. The result shows that the features of the distribution of simulated data are very similar to those of the trade data. This gives us a possible means to understand the formation of features in the trade data.