This study aims to investigate the relationships among perceived social support, objective social support and social anxiety, and the efficacy of perceived social support in Internet-based Cognitive Behavioral Therapy (ICBT). In study 1: 122 undergraduates was investigated in their social support and social anxiety at two time points between 2 months. The results indicated that there was a significant positive correlation between objective social support and perceived social support. The mediating effect of perceived social support was significant between objective social support and social anxiety, 95% confidence interval [–1.47, –0.09]. Study 2: 64 participants in different levels of perceived social support was assigned to ICBT with social support groups or non-social support groups. The results suggested that ICBT reduced participants’ social anxiety in both social support groups. Moreover, the level of perceived social support in social support groups significantly increased than that of non-social support groups, and the perceived social support level in both groups enhanced after intervention. The perceived social support interacted with the effect of ICBT with social support groups. For participants with low perceived social support, ICBT with social support groups exhibited the better intervention. For participants with high perceived social support, ICBT without social support groups exhibited the better intervention. The study further understands and clarifies the role of social support in social anxiety and provides a foundation and direction for future clinical practice and application.