Leveraging the value of heterogeneous capital and revitalizing enterprises are key strategic initiatives in China's current mixed-ownership reform. Against this backdrop, whether state-owned capital participation can effectively promote the fulfillment of social responsibility by private enterprises has become a crucial dimension for evaluating the effectiveness of mixed-ownership reform. Based on a sample of private enterprises listed on the A-share market in China from 2014 to 2023, this study explores the impact of state-owned capital participation on the social responsibility performance of private enterprises and its underlying mechanisms. The study finds that state-owned capital participation significantly improves the social responsibility performance of private enterprises. Besides, the mechanism analysis reveals that the resource effect of state-owned capital promotes the fulfillment of social responsibility by alleviating financing constraints and improving internal control. Therefore, the guiding and integrating role of state-owned capital should be emphasized in mixed-ownership reform. In this sense, by alleviating financing pressures and strengthening corporate governance, private enterprises can be encouraged to enhance their social responsibility performance, ultimately promoting higher-quality and more sustainable development.