Based on the data from China Labor-force Dynamics Survey (CLDS) in 2014,this paper conducts an empirical study on the impact of primitive social capital and new scial capital on family migration of rural-urban migrants and the mediating effect of higher level of informal employment on new social capital,which affects the family migration of rural-urban migrants. The results show that:(1) The impact of primitive social capital is obvious in the initial stage of informal employment when rural-urban migrants enter the city; (2) The new social capital has a significant positive effect on family migration; (3) The new social capital enables rural-urban migrants to obtain higher levels of informal employment and then affects their family migration. Therefore,it is proposed to motivate the rural-urban migrants to realize the extension from primitive social capital to new social capital by nurturing the new social capital. Ultimately,the family migration of rural-urban migrants is expected to be the combined effort of standardization of labor market,elimination of market discrimination and more available employment information.