This study aimed to examine the relationship between socioeconomic status and children’s psychological well-being and to investigate the mediating effect of family social capital. A sample of 19,487 school-aged children was collected from 2013–2014 China Education Panel Survey. Structural equation modeling was applied to test the hypothesized model. The results showed that socioeconomic status was not significantly related to the children’s psychological well-being. However, two indicators of family social capital, namely, parent involvement and parent–child relationship, played a complete mediating role in the direct mechanism. The theoretical and practical contributions were discussed.