State-owned listed companies in the commercial category in Shanghai and Shenzhen A-shares from 2013 to 2020 were selected as the research sample. Data related to the shareholding and delegated behavior of private shareholders among the top ten shareholders are utilized. Using a fixed-effects model, we empirically analyze the impact of private companies' participation in the mixed reform of state-owned enterprises on the value preservation and appreciation of state-owned assets at the equity level and the management right level. Explore the moderating effects of internal control and the level of market-oriented development on the above relationships. The study shows that the participation of private enterprises in the mixed reform of SOEs can effectively contribute to the value preservation and appreciation of state-owned assets, both at the equity level and at the management level. Meanwhile, the level of internal control of SOEs has a significant positive moderating effect on the relationship between the participation of private firms in SOEs' mixed reform and the value-added of SOEs' assets; The participation of private firms can effectively compensate for the impact of insufficient market development on the value-added of SOEs' assets.