"The Ballad of the Sad Café" is one of the representative works of American writer Carson McCullers, which deeply explores the master-slave dialectic relationship of the "Other" under existentialism by depicting the complex relationships among three protagonists in a southern town. In the novel, the protagonists show both a longing for and a resistance to the "Other." Marvin Macy's obsession with Amelia and his manipulation of Cousin Lymon, as well as Amelia's dependence on Cousin Lymon and her disgust towards Marvin, reflect the complex and distorted relationship between them. This relationship is not only a master-slave relationship in emotions but also a dialectical process of self-awareness and other-awareness under existentialism. The novel reveals the eternal master-slave relationship between people by showing the power struggles and psychological changes between love and being loved, control and being controlled. In this relationship, the "Other" is both the object of love and control, while the protagonists constantly seek self-identity and freedom in their relationship with the "Other." Through an in-depth study of the master-slave dialectic relationship of the "Other" in "The Ballad of the Sad Café," we can better understand the complexity of interpersonal relationships under existentialism and the eternal exploration of self and the "Other" in the depths of the human heart.