Self-efficacy is theorized as a malleable construct that can be influenced by teachers' instructions; however, limited effort has been devoted to evaluating the effectiveness of self-efficacy interventions. This study leverages a mixed methods design to evaluate the effectiveness of a self-efficacy intervention over 16 weeks among Chinese English as a foreign language learners. Data were collected from 102 secondary school students (52 and 50 students in the intervention and comparison groups, respectively). We also interviewed nine students from the intervention class. Repeated measures mixed analyses of variance were employed to check the changes in self-efficacy, intrinsic motivation, engagement, and academic achievement over time as well as to detect the differences in these aspects between the comparison and the intervention groups. Moreover, thematic analysis was adopted to handle the qualitative data. Results of the quantitative phase showed that the explicit self-efficacy supports worked effectively in promoting self-efficacy, intrinsic motivation, and language proficiency. Results of the qualitative phase triangulated the findings from the quantitative phase, illuminating the broad-spectrum effect of self-efficacy support. Implications and directions for future research are discussed.