During the planning period, China’s enterprise reform served as an effective method for achieving high-quality economic development. Furthermore, the comprehensive effects of mixed-ownership reforms profoundly influenced the innovative inputs of enterprises. By analysing cross-sectional data of 394 A-share listed companies from 2015 to 2021, this paper empirically investigates the impact of mixed ownership reforms on the innovative inputs of state-owned and private enterprises. The empirical results highlight a significant positive effect of the depth of mixed ownership reforms on state-owned enterprises’ innovation investments. In contrast, the balance degree of mixed ownership reforms negatively impacts the innovation investments of state-owned enterprises. It was found that companies with mixed ownership structures tend to increase their innovation investments more. Moreover, the introduction of non-controlling state-owned stakes in private enterprises fosters innovation investments more effectively.