Depression preventions has become a rising issue in recent years, giving the increasing prevalence of depression among children and adolescence in China. Previous studies pointed out that valued and pleasant ac-tions can improve activity level and reduce depression; moreover, Seligman et al. (2005) found that results of their positive psychology interventions （PPI） program was fully mediated by self-sustained activities. It is reasonable to assume that PPIs increase valued actions and is therefore effective, in other words, change of strength use mediates the effect of PPIs. In the current study, we designed a positive psychology group counseling (PPGC) program for pupils and conducted a longitudinal design to explore :(a) the intervention effect of this program on depressive symptoms in primary school pupils ; (b) whether the change of strength use is a mediator, and thus, a specific therapeutic factor of this program. This study recruited 414 pupils as intervention group and 171 pupils as waiting list control group, and eva-luated their depressive symptoms (using Children Depression Inventory) and level of strength use (using Children Strength Use Inventory) for 3 times (pretest, posttest, and a test after 3 months). The final sample contained 563 pupils and then the data was analyzed by WHLM 6.08 using the hierarchical linear model. The results were: 1. The PPGC significantly suppressed the increasing trend of pupils’ depressive symptoms across the time (β = -3.33，p < .001); 2. The relationship of PPGC and depressive affects was partially mediated by change of strength use, which significantly suppressed the growth trend of depressive affects across the time (β = -.56，p<.001) and was significantly enhanced by PPI program across the time (β = 1.27，p < .01), indicating that change of strength use was one of the specific therapeutic factors for PPGC program. By confirming the effects of our PPGC program for reducing depressive symptoms and enhancing level of strength use, the main findings of present study not only kept in line with previous studies of positive psychology interventions, but also further pointed out that it was an important therapeutic mechanism of PPIs to increase strength use for intervening depressive symptoms. We concluded that by encouraging children and adolescents to use their strengths in daily life, they might felt “authentic and energizing”, and thus exhibited less depressive symptoms. Our study supported the positive psychological view of mental health by empirical data, and also con-tributed to the research methodology of the application of positive psychology by introducing a technique of mediation analysis in hierarchical linear model, which had been used in evidence-based therapies such as CBT.