The improvement of farmers’ income is an important goal of the reform of collective forestland tenure and the key to sustainable management of forest resources. Based on the survey data from 18 counties of 9 provinces （districts）， this paper examines the impact of the reform of collective forestland tenure on farmers’ income and its mechanism by using difference in difference model and mediating effect test. The results show that the reform of collective forestland tenure can effectively raise farmers' income， and the reliability of the empirical results is verified by robustness test， parallel trend test and placebo test， but there are significant differences among farmers with different income levels， demonstrating the characteristic of “benefitting the poor more than the rich”； Mechanism analysis shows that the factors of motivation， livelihood strategy of farmers’ choice， forest land transfer and forest right mortgage loan play a mediating role in the impact of the reform of collective forestland tenure on farmers' income，and the reform of collective forestland tenure can increase farmers’ income by increasing the input of forestry factors， strengthening the livelihood strategy of farmers’ choice， increasing forest land transfer and the availability of credit to farmers. Heterogeneity analysis shows that the policy effect of the reform of collective forestland tenure shows significant situational dependence in terms of the differences of forest resource endowment， economic development level， human capital endowment， intergenerational degree and production structure.