Objective: To explore the relationship between 24 hour movement behavior and physical fitness in college students, so as to provide a reference for physical fitness interventions for college students. Methods: A survey was conducted from September 23 to November 12, 2022, involving 342 college students selected from one comprehensive college among Shanghai City, Hubei Province and Zhejiang Province. Accelerometers were used to measure 24 hour movement behavior, and programs specified in the National Physical Fitness Standards for Students (2014 revision) were used to evaluate physical fitness. Compositional linear regression was used to analyze the association between 24 hour movement behavior and physical fitness. The effects of compositional isotemporal substitution of 24 hour movement behavior on physical fitness were also examined. Results: The geometric means for sleep, sedentary behavior, light physical activity (LPA), and moderate to vigorous physical activity (MVPA) were 515.89, 678.88 , 196.30, 48.92 min/d, respectively, and the comprehensive physical fitness score was (73.09±8.55). The proportion of participants who passed the fitness test was 72.51%. Overall, 20.76% had excellent or good scores, whereas 6.73% failed. Compositional linear regression indicated that physical fitness was significantly positively associated with MVPA ( β =2.55) and LPA ( β = 5.88 )( P <0.05), but no significantly associated with sleep ( β =-5.18) and sedentary behavior ( β =-3.24)( P >0.05). Isotemporal substitution indicated that reallocation of 15 minutes from sleep and sedentary behavior to MVPA resulted in a 0.71 and 0.64 point increase in physical fitness, respectively. Similarly, reallocating the same duration to LPA led to a 0.50 and 0.43 point increase in physical fitness, respectively. Dose response analysis revealed that the mutual substitution of LPA with sedentary behavior and sleep had symmetry, whereas the mutual substitution of MVPA with sedentary behavior and sleep had asymmetry. The effectiveness of replacing sleep or sedentary behavior with MVPA in improving physical fitness decreased with increasing allocation time, whereas the adverse effects of replacing MVPA with sleep or sedentary behavior increased with increasing allocation time. Conclusion In the context of 24 hour movement behavior, increasing the time spent on LPA and MVPA is beneficial for physical fitness among college students. It should be recommend that college students actively transition from sedentary behavior to engaging in physical activity, with a particular emphasis on incorporating MVPA.