The private economy is a new force to promote Chinese path to modernization, and the low credit rating of private enterprises is an urgent problem to be solved. The mixed ownership reform is an important measure to leverage the integration of different equity interests to amplify the function of state-owned capital and promote economic development. From the perspective of private enterprises participating in the mixed ownership reform, this study finds that state-owned equity participation can significantly improve the credit rating of private enterprises. The impact mechanism test finds that state-owned equity participation enhances the credit rating of private enterprises by alleviating financing constraints and reducing operational risks. Further research finds that when the agency cost of management, the balance of state-owned equity, industry competition, and economic policy uncertainty are higher, state-owned equity participation has a more effective effect on improving the credit rating of private enterprises. This study indicates that the reform of mixed ownership carries huge institutional dividends and helps to achieve the healthy and sustainable development of private enterprises. It has a “1+1>2” effect on state-owned capital and private enterprises, and has important decision-making reference significance for the implementation effect of mixed ownership policies and whether private enterprises should introduce state-owned equity.