With the rise of posthumanism,posthuman discourses have become the new frontier in critical theory,providing a new theoretical perspective for interpreting classic literary works.With a theoretical reference to Deleuze's"becoming"theory and Braidotti's"posthuman"theory,this paper analyzes the becoming subject of black women in African American female writer Toni Morrison's novel A Mercy through the posthumanist lens and attempts to argue that the becoming subject of black women is the becoming posthuman subject.The black women in the novel construct their subjectivity in the non-linear temporality and present the posthuman relational identity in the nomadic act.This paper presents a new interpretation of the classic literary work with the new era's posthuman discourse,so as to broaden the critical horizon of African American women's literary works.