Objective: To explore the cross sectional and longitudinal associations between 24 hour movement behaviors and physical fitness in preschool children, and to adopt the method of equal time substitution analysis to evaluate the impact of time redistribution of different activity behaviors on physical fitness scores,so as to provide a scientific basis for promoting the health of preschool children. Methods: A total of 193 preschool children aged 3-6 years were selected from three Shanghai districts (Jing an, Baoshan, Jiading) from October to December 2023 by the stratified cluster random sampling method. The 24 hour movement behaviors were monitored via 7 day accelerometry, and physical fitness was measured according to the National Physical Fitness Measurement Standards (Revised 2023, preschool section). From October to December 2024, the follow up of physical fitness among preschool children used the same testing method. The comparison between groups was conducted by t-test. Compositional regression analyses evaluated the relationship of 24 hour movement behaviors and physical fitness among preschool children. Results: At baseline, moderate to vigorous physical activity (MVPA) time was significantly higher in boys [(84.10±25.78)min/d] than in girls [( 70.44± 25.98)min/d]; the composite physical fitness score was significantly higher in boys (71.65±8.69) than in girls (68.84±9.89), and the differences were statistically significant ( t =3.65, 2.10, both P <0.01). After adjusting for gender, age and body mass index, the results of component multiple linear regression analysis showed that MVPA time proportion was positively correlated with the composite physical fitness score at baseline among preschool children ( β =6.61), but was negatively correlated with two legged continuous hopping time at 1 year ( β =-1.12) (both P <0.05). Light physical activity (LPA) time proportion was negatively correlated with walking on the balance beam time at 1 year ( β =-4.44), and sedentary behavior (SB) time proportion was negatively correlated with the composite score of physical fitness at baseline ( β =-6.55) (both P <0.05). Isotemporal substitution analysis revealed that replacing 10 minutes of sleep (SP), SB, and LPA with MVPA increased the baseline physical fitness composite score by 0.750, 0.689 and 0.575 units, respectively; at 1 year follow up, the composite score increased by 1.440, 1.419 and 1.430 units, respectively (all P <0.05). Conversely, replacing MVPA with 10 minutes of SP, SB, and LPA,resulted in decreases in baseline physical fitness composite scores of 0.836, 0.777 and 0.669 units, and reductions of 1.613, 1.592 and 1.598 units at 1 year follow up (all P < 0.05 ). Conclusions Preschool children s 24 hour movement behaviors, especially MVPA, are closely related to physical health. Implementing appropriate strategies to increase physical activity and reduce sedentary time may improve the physical fitness of preschoolers.