In recent years, whether the mixed-ownership reform system of introducing state-owned participation shareholders into private enterprises helps to improve the environmental governance of private enterprises has been a matter of much attention and discussion. Based on data from 2007 to 2019 for Chinese A-share privately listed companies, this paper examines how the state-owned participation shareholders affect the environmental governance level of private enterprises. The results show that state-owned participating shareholder participation can improve the environmental governance level of private enterprises, and this phenomenon is more significant among industrial enterprises and enterprises in regions with a higher degree of marketization. Furthermore, there is a substitution effect between the state-owned participation shareholders, the executive team’s participation in politics, and the Party organization establishment in improving the environmental governance level of private enterprises, and the state-owned participation shareholders play a relatively larger role. In addition, the supervision effect is better when state-owned participation shareholders are from the local area and have a higher level of participation.