A survey with a sample of 418 primary school students was conducted to explore the mediating effects of family social capital in the relationship between family income and social adaptation among Chinese adolescents. Results were as follows: 1) After controlling for gender and age, family financial strain significantly contributed to adolescents’ social adaptation; 2) Consistent with the mediation hypothesis, family social capital played a mediating role in the relationship between family financial strain and academic achievement, loneliness, and peer acceptance. Therefore, the hypothesis of family investment model how family income affects adolescents’ development was confirmed.