Numerous studies have shown an alarming prevalence of anxiety disorders among youth. Prevention of anxiety disorder in this population has become crucial. Character strengths-based group intervention (CSBGI) has been considered promising for promoting adolescents' well-being and reducing anxiety, but its mechanism for reducing anxiety remains to be studied. This study examined the effects of CSBGI and the mediating role of self-efficacy between the intervention and anxiety symptoms. The CSBGI was optimized in content and form and culturally tailored for Chinese adolescents to reduce anxiety. Five classes of the 7(th) grade at an urban average middle school in Beijing (N=151, M-age=14.27, 73 females) were randomly assigned to either a CSBGI group (3 classes, N=92) or a waitlist control group (2 classes, N=59). Anxiety and self-efficacy measures were administered before, in the middle of, immediately after, and three months after the intervention. Multilevel model analysis indicated that CSBGI had a significant effect on anxiety reduction and self-efficacy improvement over time. The study also found that self-efficacy mediated the relationship between CSBGI and anxiety.