Discourse coherence is a common phenomenon in linguistic studies, and plays an important role in discourse analysis. As a common and extremely important type of language phenomena, discourse coherence has drawn more and more scholars' attention, but they emphasis on partial discourse coherence, paying a little attention to holistic coherence. The paper is an attempt to explore the mechanism of discourse coherence within the framework of Mental-Model based stereotypical relation theory, and proves that it is the rule of similarity and proximity that leads to the discourse coherence.