Taking Chinas A share listed companies from 2012 to 2016 as research samples, this paper constructs an intermediary model with agency cost as the mediating variable, and explores the influence of internal control on agency cost and corporate social responsibility, as well as the intermediary role of agency cost. The research results show that internal control has a positive impact on corporate social responsibility, and agency costs play a part of intermediary role between internal control and corporate social responsibility. Namely, internal control can promote the fulfillment of corporate social responsibility by reducing agency costs. Further research finds that compared with state owned enterprises, agency cost has a more significant mediating effect between internal control and corporate social responsibility in non state owned enterprises.