Based on CLHLS data in 2018, this paper empirically analyzes the effect of participating in social activities on the mental health of the elderly in China by using Oprobit model. The findings are as follows: Firstly, compared with the elderly who have not participated in social activities, participating in social activities is helpful to reduce the depression of the elderly and improve their mental health. Secondly, the effect of participating in social activities on the mental health of the elderly is heterogeneous. For the elderly with old-age insurance, female groups and rural hukou, as well as the western region, the effect of social activities on improving the mental health level is more obvious. Thirdly, taking the depression score as a substitute variable for empirical analysis, it proves once again that participating in social activities reduces the depression score of the elderly, and proves that the mental health improvement effect of social activities of the elderly is very significant and stable. Fourthly, through the analysis of intermediary effect, it is found that participating in social activities can improve the mental health of the elderly by influencing their existing life evaluation. In order to accelerate the construction of healthy aging in China, it is urgent to continuously strengthen the health literacy of the elderly, and encourage them to actively participate in social activities based on life health, so as to effectively improve the mental health problems of the elderly.