Chinese college students’ physique were significantly decreased in recent years. Their physique enhancement campaigns urgently need to be implemented. In mobile age, Mobile fitness applications are widely used to enhance people’s physical activity participation. Most mobile fitness application related studies were carried out in development countries. And little causality study was done. WeChat is the most popular mobile social network tool which contains the physical fitness for the Chinese population. Does WeChat fitness service adoption really enhance college students’ physical activities? This study uses a quasi-experiment design (N = 430) and develops an extension model based on the Theory of Planned Behavior (TPB) to interpret this hypothesis. Results show that WeChat fitness service adoption has a positive effect on college students’ physical activities and mechanism of WeChat fitness service adoption behavior is quite diversified. This study suggests that WeChat fitness service is worthy to popularize to college students population.