Eyeing the huge potential mHealth market in China, developers both inside and outside of China have created an increasing number of fitness mobile applications with Chinese-language user interfaces. The present study analyzes the content of those fitness mobile apps (N = 177), with a particular focus on their behavior change techniques and relevant theoretical mechanisms. It finds that three theoretical mechanisms, modeling/observational learning, self-regulation, and social comparison/social support, are prevalent among fitness mobile apps with Chinese-language user interfaces. Moreover, based on the configurations of the behavior change techniques, three distinct clusters are identified: "instructional apps" (N = 75), "self-regulation apps" (N = 58), and "triathlon apps" (N = 44). Among them, "triathlon apps" equipped with technical features reflecting all three theoretical mechanisms are found to be the most popular among users. This suggests the usefulness of health behavior change theories in promoting physical activity via mobile apps in that the inclusion of more theoretical content in the app design enhances the app's effectiveness. More theoretical and practical implications are also discussed.