Based on the data of sample companies of China’s A-share listed companies from 2014 to 2018, this paper empirically studies the moderating effect of external governance environment from three dimensions of government intervention, financial development and legal protection on the relationship between executives’ overconfidence and over-investment. The results show that the degree of government intervention is positively correlated with the over-confidence of the executives and over-investment. The stronger the degree of government intervention, the higher the possibility of the over-confidence of executives leading to over-investment. Meanwhile, financial development and legal protection have a negative correlation with executive overconfidence and over-investment. The higher the level of financial development and legal protection is, the less possibility that executive overconfidence will lead to enterprise over-investment. Finally, from perspectives of the external governance environment of government intervention, financial development and legal protection, this paper puts forward some policy suggestions to restrain the overconfidence of listed company executives and restrain irrational over-investment.