介词作为一个古老的词类在语言学领域的研究有着很长的历史，而且方法多样。对于介词的语义扩展机制也有不同的研究方法。很多相关研究都以传统语法、结构语法和功能语法为基础。但是这些方法都不能完全揭示介词语义结构的实质。1980年莱考夫与约翰逊的《我们赖以生存的隐喻》从隐喻角度探讨了语言的本质，并证明了语言和隐喻认知结构的密切相关性。其后出版的约翰逊、兰盖克、莱考夫的语言与认知研究专著进一步将心理学、神经学、人类学和计算科学融为一体，建构了认知语言学，它强调人类经验和认知能力的重要性，由此开辟了一条新的研究方向，并对许多语言现象做了具有说服力的诠释。从理论上讲，对于语言范畴的概念基础和经验基础的分析在认知语言学中是非常重要的。语言就是一种认知范畴体系。世界是由无数不同的物质、形状和颜色构成的。我们将这个客观世界进行分类的思维过程就是范畴化过程，范畴化的结果即认知范畴。而介词的每一个意义就反映一个认知范畴。莱考夫认为一共有四种认知模式：命题模式、意象模式、隐喻模式和转喻模式。它们被称为理想化认知模式。而其中的意象模式和隐喻模式可以用来分析英语空间介词的语义扩展机制。本文作者从理想化认知模式出发，整合了一个专门用于分析介词on的认知模式，并由此形成了介词on的语义网状图。在对介词on进行分析的过程中，作者主要通过选自语料库中的句子来说明介词on是如何从原型意义扩展到其他意义的。介词on的原型意义指的是其空间意义，其扩展意义主要指表示时间、抽象位置、状态或者活动、数量或者范围、方向、附着、基础和比较的意义。此外，当介词on被当作副词使用时，也是一种从静态意义域向动态意义域的映射，因此，副词on也是介词on的一种扩展意义。通过对介词on的语义扩展机制进行分析，我们可以更深入地理解该介词的意义。因此，本项研究对英语词汇的教与学是有用的，对翻译学和词典学也不无启发。

As an old word-class, prepositions have been studied much in linguistics for a long time. Different approaches have been applied to exploration of lexical meaning extension of prepositions. Some researches have been carried out on the basis of traditional grammar, structural grammar or functional grammar. They can not fully reveal the true nature of the semantic structures of prepositions. The book Metaphors We Live by (Lakoff & John 1980) discussed the nature of language from the perspective of metaphor, which proved a close relation between language and cognitive structure of metaphor. Later, the influential works of Johnson, Langacker and Lakoff further brought together what is known about the mind from many academic disciplines: psychology, neuroscience, anthropology, computer science, and the related disciplines. Cognitive linguistiics emphasizes the importance of human experience and cognitive capability and gives some new and persuasive explanations to many linguistic phenomena. Methodologically speaking, the analysis of the conceptual and experiential basis of linguistic categories is of primary importance within cognitive linguistics; it primarily considers languages as a system of cognitive categories. The world consists of an infinitive variety of objects with different substances, shapes, and colors. We have been always classifying those objects. This mental process of classification of the physical world is now commonly called categorization, and its products are the cognitive categories. And each sense of a preposition reveals a category. According to Lakoff (1987), there exist four cognitive models: propositional model, schematic model, metaphoric model, metonymic model. These four models are called Idealized Cognitive Models (ICMS). Image schema and metaphor are two main approaches to spatial preposition cognition. Image schemata and their metaphorical extension have been used to explain the mechanism of the lexical meaning extension of spatial prepositions. Based on the ICMS the present author makes an integrated model for this thesis. According to this model a network of the preposition on is formed. Sentences selected from Oxford Advanced Learner’s English-Chinese Dictionary and from the corpus in the School of Foreign Languages and Literatures are used to illustrate different senses of the preposition on. They are: the spatial sense or the prototypical sense, the temporal sense, the sense of abstract or figurative position, the sense of state or activity, the sense of quantity or scale, the sense of direction, the sense of attachment, the sense of basis, the sense of comparison and the dynamic sense.Through such an analysis of on we can get a deeper understanding of the meanings and usages of this preposition. Therefore, the present study has some significant implications for English vocabulary learning and teaching as well as translation and lexicology.