Objective: To study the association between 24 h activities and Fundamental Motor Skills (FMS) among children with autism spectrum disorder (ASD) using compositional data analysis, and the expected changes in FMS after isochronous substitution of each activity, in order to provide reference basis for improving FMS levels in children with ASD. Methods: From October 2023 to April 2024, a total of 301 children with ASD aged 6-10 from 7 special education schools in Jinan, were investigated by cluster random sampling, and 24 h movement behaviors were calculated based on accelerator data. Test of Gross Motor Development- 2 was used to assess FMS. R software was used to perform the descriptive statistical, multiple linear regression and isochronous substitution analyses. Results: The proportion of moderatevigorous physical activity (MVPA) in children with ASD was positively related with FMS scores, locomotor, and object control skills ( β=12.42, 6.32, 6.10, P <0.01). Reallocating 15 min from sleep (SLP) to MVPA resulted in respective increases of 3.66, 1.91, and 1.75 points in FMS scores, locomotor skills, and object control skills ( P <0.05). Reallocating 15 min from sedentary behavior (SB) to MVPA resulted in respective increases of 3.72, 1.88 , and 1.83 points in FMS scores, locomotor skills, and object control skills ( P <0.05). Reallocating 15 min from light physical activity (LPA) to MVPA resulted in respective increases of 3.32, 1.57, and 1.74 points in FMS scores, locomotor skills, and object control skills ( P <0.05). Moreover, reallocating 15 min from SB to LPA resulted in an increase of 0.28 points in locomotor skills ( P <0.05). Dose response analysis revealed that substitution of MVPA for SLP, SB, and LPA in children with ASD enhanced their FMS levels, and their substitution was asymmetrical; and substitution of LPA for SB enhanced locomotor skills level. Conclusions Among the 24 h movement behaviors, increasing the time spent on MVPA and LPA have positive impacts on the FMS of children with ASD. Schools and families should optimize the allocation of 24 h activity time in children with ASD, so as to promote the improvement of FMS levels of children with ASD.