Discourse is the basic resource of English teaching, and discourses in textbooks are an integral part of students’ learning content. English Curriculum Standards for Senior High Schools states that teachers should probe into the discourses and grasp the core content of teaching. The in-depth interpretation of textbook discourses is particularly important for the implementation of in-depth teaching. Based on extended cognition, this study puts forward three levels of discourse: logical, contextual and situational. Accordingly, when interpreting a discourse, teachers are also required to take the logic of knowledge, the context of the text and the situational nature of the language as an organic whole, go beyond the surface structure of knowledge, enter the teaching of deep structure, and lead the teaching to the depths of students’ thinking and emotion.