This paper mainly studies the thought of “nothing” in Sater's philosophy and points out the relationship between “nothing”, the philosophical basis of freedom, and Sater's freedom theory. In Satre's philosophy, being is prior to “nothing” but would be meaningless without “nothing”; however, man pursues the being in the unstable state, which causes useless warmth of life. Moreover, the essence of man is “nothing”, the basis of freedom, therefore, man can enjoy unlimited freedom.