Aristotle’s Poetics is the first systematic work to defend "poetry" in the western history of the theoretical study of literature and art,it is also the originator of the western theories of poetics.Aristotle chooses tragedy as the representative for art,rationalizing,systematizing and philosophizing it to justify art.This essay attempts to clarify the mechanism of Aristotle’s theory of tragedy,and to investigate the formations and transformations of the poetical concepts of "mimesis" and "art".