The rise of theory in the 1980s benefited greatly from the emergence of many excellent literary theory textbooks. Through a comparative analysis of the most influential literary theory textbooks in the past few decades, this essay explains their different understandings of the function of theory. Some textbook authors mainly understand theory as a kind of critical discourse, while others view it as a set of critical tools. The different understandings of the essence and function of literary theory led to varying compilations of textbooks. Literary theory is different from scientific theory. While it cannot be uprooted from practice, it is not an induction from critical practices, but a reflection and critique of it. Though theory should shed light on the practice of criticism, its function is not limited to this, but to make our literary research activities more reflective.