The Third Plenary Session of the 20th CPC Central Committee announced to “ build a high-level socialist market economic system”, in which private economy, as an important component of China’s socialist market economy, plays a necessary role. Based on the theoretical framework of Marxist political economy, this paper chooses fulfillment of social responsibility as the research focus, clarifies the important role of China’s private economy in the construction of the socialist market economy in three aspects: historical development, theoretical logic and path selection. After analyzing the behavior characteristics of private enterprises in the socialist market economy, we concluded that the environmental advantages of the socialist system enable private enterprises to unify their own economic interests and the social goals in their development, and they can change the internal production relations of the enterprise through employee stock ownership and other ways. In addition, private enterprises can play an important role in the construction of a high-level socialist market economy system by innovation of science and technology and production relations.