Abstract: The mixed ownership reform aims to increase the efficiency of state-owned capital allocation and improve the corporate governance of state-owned enterprises. It is the current focus and key of China's state-owned enterprise reform. In October 2022,the 20th National Congress of the Communist Party of China (CPC) pointed out that “deepening the reform of state-owned capital and state-owned enterprises,accelerating the optimization of the state-owned economic layout and structural adjustment,promoting the state-owned capital and state-owned enterprises to become stronger and better,and enhancing the core competitiveness of enterprises”. Mixed ownership reform has continued to advance at different levels of the state-owned economy. With the continuous strengthening of mixed ownership reform,non-state capitals have taken shares in state-owned enterprises,which has brought significant impacts on the governance structure,business mechanism and operational efficiency of state-owned enterprises. Based on this,academics have explored the economic consequences of the mixed ownership reform of state-owned enterprises (SOEs),focusing mainly on the economic efficiency of SOEs in terms of investment activities,financing activities and enterprise performance. In recent years,how to promote green economic and social development has attracted worldwide attention. The overall task of China's ecological civilization in the new era is to promote green development and harmonious coexistence between human beings and nature. However,existing researches on the consequences of mixed ownership in SOEs have paid more attention to the economic development of the enterprises,but less attention to the impact of the investment on environmental protection. The impact of mixed ownership reform of state-owned enterprises on environmental issues has been an important topic of widespread concern in both theoretical and practical circles. Based on this, the paper explores how the degree of mixed ownership affects the environmental investment of state-owned enterprises and how local government environmental regulation influences this relationship. Focusing on the above issues,this paper uses the data of Chinese A-share state-owned listed companies as a sample to systematically investigate the relationship between mixed ownership and environmental protection investment. We find that the degree of mixed ownership is negatively correlated with environmental protection investment. The greater the diversity of mixed ownership and the higher the degree of integration,the less investment in environmental protection. We further consider the impact of the new Environmental Protection Law,which was implemented on 1 January 2015,and find that the relationship between the degree of mixed ownership and environmental protection investment remains negative before and after the implementation of the institution. This negative relationship is significantly alleviated after 2015,reflecting the importance of government environmental regulation. Therefore,we then explore the impact of government environmental regulation to further confirm the mitigating effect of government environmental regulation on the negative relationship between the degree of mixed ownership and environmental investment. Further analysis indicates:distinguishing the types of environmental protection investment,the degree of mixed ownership has an inhibitory effect on different types of environmental protection investment; distinguishing the degree of industry pollution,the degree of mixed ownership has a stronger inhibitory effect in heavy polluting industries; distinguishing the participation of mixed entities,the higher the degree of foreign and private equity participation,the lower the investment in environmental protection; distinguishing the level of state-owned enterprises,the degree of mixed ownership in central state-owned enterprises has less impact. The conclusions of the paper remain unchanged after a series of robustness tests including the difference-in-differences model,Heckman two-stage model,firm fixed-effects model,adding control variables,and changing the environmental regulation measure. The finding of this paper has the following contributions. First,previous studies on mixed ownership in SOEs have mainly focused on corporate performance,investment decisions,risk-taking,etc.,and few papers have paid attention to the impact of the degree of mixed ownership on environmental protection. Although most scholars have found that the degree of mixed ownership promotes the performance growth of SOEs,the performance growth does not mean the growth of environmental protection inputs,and it is even possible that the performance growth is at the expense of environmental protection inputs. This paper examines the impact of the degree of mixed ownership on environmental protection and broadens the study of the economic consequences of the degree of mixed ownership. Second,previous research on environmental protection has focused on environmental regulation,government regulation,social media and so on. Little literature has focused on the impact of changes in own equity structure. SOEs hold a large amount of economic resources and occupy a monopoly position in important industries affecting the environment,and the efficiency of SOEs in environmental protection largely determines the ultimate effect of environmental protection in China. This paper provides a new perspective for the study of environmental protection and highlights the importance of government environmental supervision,and also provides inspiration on how to synergistically improve mixed ownership and environmental protection.