This study systematically examines the effects of mobile health (mHealth) on physical activity in a Chinese fitness app (WeRun). Drawing on the theory of reasoned action, we identified several psychosocial mechanisms for understanding mHealth effects and unpacked these mechanisms through a survey with 403 WeRun users. We also collected self-tracked behavioral data on the daily step counts of the participants over the previous week of the study. As predicted, exercise intention was a strong predictor of self-tracked physical activity. The results provide compelling support for the social support, social comparison, and attitudinal mechanisms in motivating physical activity via fitness apps. For social support features, informational support was indirectly associated with stronger exercise intention via the mediation of attitudes towards exercise. The esteem support was directly associated with stronger exercise intention. For social comparison features, upward and downward comparisons were associated with exercise intention via attitudes towards exercise but in opposite directions. The findings are discussed for theoretical implications for understanding mHealth behaviors and practical implications in mHealth design.