With the internalization of the international market and the formation of the pattern of international domestic competition, how to focus on the main business, cultivate the core competitiveness and achieve high-quality development in the competition of the market economy has become a problem to be solved. Taking Shanghai and Shenzhen A-share non-financial private listed companies from 2013 to 2021 as the research object, this paper empirically examines the impact of "Reverse Mixed Reform" on the diversified operation of private enterprises and its action path by constructing A multi-period differential model. The findings are as follows: (1) "Reverse Mixed Reform" can significantly inhibit the diversification of private enterprises, and this conclusion is still valid after a series of robustness tests and endogeneity treatment. (2)By reducing external transaction costs and alleviating agency problems, "Reverse Mixed Reform" weakens private enterprises' motivation for diversified expansion, thus forcing them to rationally allocate their own resources and divest inefficient and non-main businesses.(3) When industrial state-owned capital and state-owned shares of central enterprises participate in private enterprises, the "Reverse Mixed Reform" has a more significant inhibitory effect on the diversified operation of private enterprises. (4) "Reverse Mixed Reform", while restraining the blind and diversified expansion of private enterprises, can also promote private enterprises to return to the main business, increase R&D input and improve innovation output. This study provides empirical evidence for private enterprises to focus on main business management, cultivate core competitiveness of main business and achieve high-quality development.