Based on the background that state owned enterprises vigorously carry out the reform of mixed ownership, this paper makes an in depth study of the influence of mixed ownership on the efficiency of mergers and acquisitions of state owned enterprises and its mechanism. It is found that the implementation of mixed ownership reform and the increase of the proportion of non state owned shares are conducive to the realization of “less but better” M&A activities of state owned enterprises, that is, to reduce the possibility of M&A, but the selected projects can obtain high quality M&A performance and show high M&A efficiency on the whole. Moreover, this promotion effect is more significant when non state owned large shareholders participate in the corporate governance of state owned enterprises. Further research shows that when the mixed reform state owned enterprises are in competitive industries or belong to local state owned enterprises, the mechanism of non state owned large shareholders governance effect is more prominent. The conclusion is helpful to deepen the reform of mixed ownership, provides empirical evidence for state owned enterprises to use market forces to “mix” equity, “corporate” governance and provides empirical evidence for the realization of state owned enterprises value creation.