Relevance Theory from the perspective of cognitive psychology argues that human communication is an ostensive-inferential process, and emphasizes the function of the optimal relevance for communication. In this sense, reading comprehension could be considered as a kind of communication in which the writer manifests his/her communication intention and the reader infers from the discourse codes; thereinto, the optimal relevance between the textual information and the reader's cognition is essential to the optimal contextual effects. This paper sets out to discuss the explanatory power of relevance theory to reading comprehension, with focus on differences among readers with different reading abilities in grasping the optimal relevance with the discourse and the cognitive context. Through the reading teaching experiment undertaken for a semester, the result shows that the application of relevance theory to reading by pinpointing reading purpose, setting reading tasks and constructing cognitive contexts benefits greatly to students' inference capability, hence their reading ability, which is also instructive for the teaching mode of reading courses.