Objective: To investigate the relationship between 24 hour activity behaviors and fine motor development in preschool children, and to simulate the effects of reallocating time among different activity behaviors on fine motor development using isotemporal substitution analysis. Methods: From March to July 2022, a stratified cluster random sampling method was used to recruit 447 preschool children aged 3-6 years (254 boys and 193 girls) from Pingxiang City, Jiangxi Province. The 24 hour activity behaviors in preschool children were measured using ActiGraph wGT3-BT accelerometers and subjective sleep reports. Fine motor development was assessed using the Gesell Developmental Schedules. A component linear regression model was employed to analyze the impact of 24 hour activity behaviors on the development of fine motor activities, and the potential effects of adjusting activity behaviors were simulated through an isochoric substitution model. Results: The daily durations of sedentary behavior (SB) was ( 572.92 ±102.96) min (accounting for 39.79% of 24 h), the duration of light physical activity (LPA) was (131.21± 38.11 ) min ( 9.11% ), the duration of moderate to vigorous physical activity (MVPA) was (65.61±22.21) min (4.56%), and sleep duration was (670.65±57.58) min (46.82%). Sleep composition was positively associated with fine motor development ( β =2.74), while MVPA ( β =-0.84) and SB ( β =-1.93) compositions were negatively associated with fine motor development (all P <0.01). Isochoric substitution analysis showed that sleep had positive effects on the development of fine motor skills when replacing other activity behaviors (all P < 0.05), with the substitution effect for MVPA being the most significant and gradually increasing with the duration of substitution (60 min: β =28.66); sleep replacement of SB and LPA also showed positive effects (60 min: β =4.25, 2.00) (all P < 0.05). On the contrary, the substitution of sleep with MVPA showed negative effects (60 min: β =-7.86), and the substitution of LPA and SB with MVPA also showed negative effects (60 min: β =-5.65, -3.40) (all P <0.05). Conclusions The overall composition of 24 hour activity behaviors is associated with fine motor development in preschool children, with sleep playing a crucial role. Ensuring adequate sleep and optimizing the structure of activity behaviors may effectively promote the development of fine motor skills in preschool children.