Mr. Sammler’s Planet is the major masterpiece of American writer Saul Bellow. In the form of stream of consciousness, the novel presents the protagonist Mr. Sammler’s traumatic memories of the Holocaust and social reality in New York in the 1960s. From the perspective of Sartre’s existentialism, the paper reveals the absurdity of the counterculture and moral crisis in the American society at the time, and shows the consciousness predicament of people in the being-for-others as well as the distorted personality and sociopathic madness that modernity invites in the being-for-itself people. People pursue unrestricted absolute freedom, and the emptiness of existence gives rise to the desire to exist while granting rootless freedom. The novel enlightens people on how to maintain their authentic existence in absurd situations and break through the confusion and helplessness by means of the free choice with a sense of responsibility, which are the philosophical implications of the work.