The aging population in rural China is becoming increasingly severe, and the traditional elderly care model is no longer able to solve the current problems of elderly care mainly based on care needs. Mutual assistance for the elderly is highly anticipated. Based on the micro data from the China Health and Retirement Longitudinal Study (CHARLS) and employing the regression discontinuity design method, this paper studies the spillover effect and mechanism of the New Rural Social Pension on the mutual assistance for the elderly. The empirical results show that  the mutual assistance behavior of the rural elderly people who receive New Rural Social Pension has increased by 52.4 percentage points, and this conclusion still holds after a series of robustness tests. Reducing agricultural labor time and enhancing happiness are important mechanisms for the New Rural Social Pension to promote mutual assistance among the elderly people in rural areas. Further heterogeneity analysis reveals stronger promotion effects  on the mutual assistance behavior among the elderly with poorer health, at lower income levels or in rural areas in the central and western regions. The implication of this paper is that improving the elderly care security system in rural areas of China is conducive to the construction of the emerging elderly care model of mutual assistance, thereby contributing to the realization of China's vision of “active aging”.