Objective: An isochronous substitution model was established to explore the association and substitution effect between college students 24 hour activity behavior and physical health, so as to provide specific activity behavior suggestions for college students to improve their physical health. Methods: A stratified random cluster sampling method was used to conduct physical fitness tests and 24 hour activity behavior surveys among 2 794 college students in 12 colleges and universities in Tianjin.Time spent on sedentary behavior(SB), light intensity physical activity(LPA), moderate to vigorous physical activity(MVPA) and sleep(SLP) time. The isochronous method of components was used to explore the relationship between 24 h activity behavior and physical health. Results: Except for 50 m running, MVPA was negatively correlated with BMI Z ( β =-0.62, P <0.05), but positively correlated with other physical fitness indexes ( β =0.34~274.23, P <0.05). LPA was not associated with lung capacity, sitting forward flexion and 50 m running, and negatively correlated with other physical fitness indexes ( β =-14.30- -0.19, P <0.05). SB was negatively correlated with most physical fitness indexes ( β =-11.57- -0.33, P <0.05), but positively correlated with BMI Z ( β =0.45, P < 0.05 ). In addition to lung capacity, SLP was positively correlated with BMI Z , total physical fitness score,1 minute sit-ups, pull ups, 800/1 000 m running, sitting forward flexion, and 50 m running ( β =0.27-11.21, P <0.05), but negatively correlated with long jump ( β =-0.10, P <0.05）. Isochronous substitution showed that the adverse effects of 30 min/d SB and LPA substitution of MVPA were much greater than the beneficial effects of MVPA substitution for corresponding behaviors (total physical score: SB, -0.58 vs 0.47 points; LPA, -0.50 vs 0.38 points). Conclusion MVPA and SLP have been found to have a positive effect on physical fitness among college students. Therefore, in the process of improving the physical health of college students, ensuring adequate sleep, improving MVPA and reducing SB as much as possible may be one of the effective methods.