Investment decisionmaking is the core of enterprise financial decisionmaking, therefore, how to improve corporate investment efficiency and reduce investment distortion has attracted wide attention. This article takes the Shanghai and Shenzhen Ashare listed companies from 2003 to 2017 as a research sample to analyze the relationship between CEO overconfidence, accounting conservatism, and corporate investment efficiency. The study finds that accounting conservatism plays an intermediary role in CEO overconfidence and investment efficiency. CEO overconfidence will inhibit accounting conservatism and further reduce the investment efficiency of the enterprise. The intermediary effect of CEO overconfidence only exists in overinvestment companies and high growth enterprises, it is not significant for enterprises with insufficient investment. This article also proposes corresponding policy suggestions from the level of enterprises and regulatory agencies.