Taking state-owned enterprises (SOE) listed on the A-share from 2011 to 2021 as the sample, we test the mechanism by which mixed ownership reform affects the investment efficiency of SOE. The results show that mixed ownership reform can effectively improve the investment efficiency of SOE, and the level of corporate governance plays a partially intermediate role between them. Further analysis finds that higher economic policy uncertainty weakens the positive effect of mixed ownership reform on the investment efficiency of SOE. Heterogeneity analysis finds that the promotion role of mixed ownership reform on the investment efficiency of SOE is more significant in the absence of establishing a liability insurance system for directors and officers, and in local SOE. The conclusion has important practical significance for promoting the mixed ownership reform and long-term development of SOE.