The purposes of the current study were to explore the applicability of the Health Action Process Approach (HAPA) model for interpreting changes in physical exercise behavior and to examine the key determinants of changes in physical exercise. The participants were 639 rural middle school students in Sichuan province, China, who did not perform physical exercise. Three surveys and two interventions were completed in the same participants within 1.5 years. The HAPA model elements and physical exercise were estimated by a self-reported questionnaire. The results showed that 158 students (24.7%) formed a habit of physical exercise. The structural equation model for the pre-intention stage and behavior stage showed acceptable goodness of fit. Outcome expectancies (=0.136, 0.014) and action self-efficacy (=0.314, 0.001) directly predicted intention of physical exercise, the latter directly predicted physical exerciseplanning (=0.537, <0.001), andplanning subsequently predicted physical exercise (=0.324, <0.001). Maintenance self-efficacy indirectly predicted physical exercise through planning (95%CI: 0.014, 0.053). The findings suggested that the HAPA model was a very useful tool for predicting changes in physical exercise behavior, as this model explains the process of changing physical exercise habits and reveals the weak link in such behavioral changes among Chinese rural adolescents.