Social supports are the perceived respect, love and help from others, such as parents, teachers or peers. Currently, the mechanism how the social supports have impact on achievement is still open to question. The aims of the study was to explore the influence of different social supports (specifically, from parents, teachers and peers) on academic achievement and to test the mediating effect of academic self-concept between them. Meanwhile, we also checked if the mediating effect varied between boys and girls. 2519 eighth-grade students from 9 junior middle schools in Hangzhou, China were required to finish social supports and academic self concept scales and standard Chinese and Maths achievement tests. The stepwise regression was used to determine the most important support resources among parents, teachers and peers. The result revealed that peers support could predict academic achievement significantly (β=.36, p<. 001) and the parents’ and teachers’ supports could not enter the equation, which were stable within both girls and boys. Given that the parents’ and teachers’ support could not predict academic achievement when controlling the peers’ support, we only consider peers’ support when building mediating model. We built the regressions of academic self concept on peers’ support and the regression of academic achievement on academic self concept and peers’ support separately. Also, we repeated the mediating effect analysis within boys and girls group separately. The results showed that (1) peers’ support could predict academic achievement significantly (β=.72, p<.001) but it could not predict academic achievement (β=.003, p>.05) when controlling academic self-concept; (2) no matter for boys or for girls, the peers’ support could predict academic achievement significantly (βmale=.71, p<.001; βfemale=.79, p<.001). However, when controlling academic self concept, the peers’ support could not predict academic achievement for boys (β= -.06, p>.05) but could predict academic achievement for girls (β=.10, p<.01). Conclusions: (1) For Eighth Graders, peers’ support was more important than parents’ and teachers’ support. (2) Academic self-concept was the mediator between social support and academic achievement. (3) For boys, the effect of social support on school achievement was fully mediated by academic self-concept; for girls, the effect of social support on school achievement was partially mediated by academic self-concept.