Computational Thinking (CT), one of the necessary and fundamental skills for students in the 21st century, is gaining more attention in K-12. The premise of integrating CT with other subjects is to promote school teachers with high self-efficacy in CT. Towards this, we developed a three-week Scratch online professional development(PD) course based on MIT CT framework, then implemented an online training among 84 primary and secondary school teachers from different subjects including Chinese, Mathematics, English, and Computer Science. To examine the effect of its impact on teachers’ self-efficacy in CT, a quasi-experiment method of a single group pretest-posttest was conducted, and quantitative and qualitative data were collected from scales, artifacts, and interviews. The results revealed that the online PD course significantly improved teaching self-efficacy in CT of these teachers. The findings indicated that online PD integrated with MIT CT framework through block-based programming was a potential approach to effectively promote teaching self-efficacy in CT for primary and secondary school teachers.