Objective: To explore the relation between procrastination and physical activity in college students and test whether grit mediated, age and gender moderated the process. Participants: 610 college students (aged 20.07 ± 1.53; 45.2% males) in Shanghai. Methods: Procrastination, grit and physical activity were tested respectively by International Procrastination Scale, Grit-S scale and International Physical Activity Questionnaire. Results: Correlation analysis showed that procrastination, grit and physical activity were related in pairs. Procrastination was negatively correlated with physical activity. Grit mediated between procrastination and physical activity. Females showed stronger moderation effect from grit to physical activity than males, while the effect between procrastination and grit was more significant for the younger. Conclusion: The moderated mediating model provided a new strategy of improving personality traits which contributed to lack of physical activity. More psychologically relevant measures should be taken in reducing procrastination to improve physical activity via improving grit.