The purpose of this paper is to analyze the meanings of English preposition within the framework of Cognitive Grammar, and to examine the application of their basic meanings in various situations. In this paper, I analyze at, on, and in as basically profiling the spatial relation between two things, trajector and landmark. Besides the basic meanings, they can also profile a metaphorical abstract as its extended usage. Their own extended meanings are easily understood when we realize their basic meanings. When the basic meanings are set up in spatial concepts, the regularity in using these prepositions can be applied consistently not only in spatial relations but also in abstract relations. Therefore, anyone who wants to be a interpreter or translator should be good with prepositions. Also, we had better know the framework of the Cognitive Linguistics.