<p>This study investigated longitudinal trajectories of screen time and outdoor physical activity from ages 6 to 48 months, and their independent and interactive associations with children's executive function (EF) and behavioral problems. This study included 1414 children from a prospective cohort study, the Shanghai Birth Cohort, enrolled between 2013 and 2016. Parents reported their children's screen time and outdoor physical activity at ages 6, 12, 24 and 48 months. Children's EF and behavioral problems were assessed at age 48 months using the Behavior Rating Inventory of Executive Function–Preschool Version (BRIEF-P) and the Strengths and Difficulties Questionnaire (SDQ), respectively. We identified 3 trajectories of screen time: low stable, medium increasing, and high increasing; and 2 trajectories of outdoor physical activity: low activity and high activity. After adjusting for confounders, we found a significant trend across screen time trajectories, with higher screen time trajectory linked to greater EF difficulties and behavioral problems. Furthermore, children in the low outdoor physical activity trajectory exhibited more EF difficulties and behavioral problems than those in the high activity trajectory. While no significant interaction effects between screen time and outdoor physical activity trajectories were observed, the negative impact of high screen exposure appeared more pronounced in children with low activity trajectory. Higher screen time and lower outdoor physical activity during early years were associated with worse EF and behavioral problems, and sufficient outdoor physical activity may partly mitigate the negative effects of increased screen exposure.</p>