The reform of mixed ownership of private enterprises is an important way to boost high-quality economic development, and also an effective means to broaden financing channels, alleviate financing constraints and optimize corporate capital structure. However, relevant researches are relatively insufficient. By using the M&A data of private listed companies from 2014 to 2018 to test the mechanism of its effect on the speed of capital structure adjustment, it is found that the cross-ownership M&A of private enterprises can promote the speed of capital structure adjustment, and it is realized mainly by reducing the financing constraints and agency costs of enterprises, and the financing constraints and agency costs play a part of the intermediary effect. The research conclusion not only provides new ideas and methods for alleviating the financing constraints and agency problems of enterprises, but also has positive significance for protecting and paying attention to the development and integration of non-public enterprises, promoting them to give full play to the advantages of private capital and contributing to the construction of mixed ownership economy.