The objectives of the survey were to explored the associations of the 24-h movement behaviours (MB) with executive functions (EFs) and quantified the predicted changes in EFs following allocation of time among behaviours. In the cross-sectional survey, 135 preschoolers (3 ~ 5 years) were enrolled. Physical activity (PA) and sedentary (SED) time were objectively measured employing an ActiGraph GT9X. Sleep time was reported by parents. EFs were assessed using the iPad-based Early Years Toolbox which is a collection of computerized tasks consisting of brief tasks assessed from games administered and scored according to protocol. To explore the associations of the 24-h MB with EFs, compositional multiple linear regression was employed. To quantify the predicted changes in EFs following allocation of time among behaviours, compositional isotemporal substitution was used. Moderate-to-vigorous physical activity (MVPA) was positively related to cognitive flexibility. Replacing sleep or SED with MVPA was associated with positive changes in cognitive flexibility. When MVPA was replaced with sleep or SED, the predicted detriments to cognitive flexibility were larger than predicted benefits of replacing sleep or SED with MVPA. The findings highlight the key role of intensity of PA for preschoolers' EFs and the importance of meeting recommended levels of MVPA.