Abstract: Objective: To explore the short-term and long-term within-person mechanisms of physical exercise on early adolescents' internalizing and externalizing problems. Methods: The random intercept cross-lagged panel model was employed in Study 1, and one-year longitudinal data from 1 431 middle school students was analyzed. In Study 2, the dynamic structural equation model was used to analyze 15-day diary data from 163 middle school students. Results: 1) In Study 1, T1 physical exercise negatively predicted T2 internalizing problems and externalizing problems, and T2 physical exercise negatively predicted T3 internalizing problems; T2 internalizing problems positively predicted T3 externalizing problems, and T2 externalizing problems positively predicted T3 internalizing problems, a bidirectional predictive relationship was found among them; T1 physical exercise reduced T3 externalizing problems through decreasing T2 internalizing problems, and T1 physical exercise reduced T3 internalizing problems through decreasing T2 externalizing problems. 2) In Study 2, physical exercise on the previous day negatively predicted internalizing problems and externalizing problems on the following day; internalizing problems on the previous day positively predicted externalizing problems on the following day, and externalizing problems on the previous day did not predict internalizing problems on the following day; physical exercise on the previous day reduced externalizing problems through decreasing internalizing problems. Conclusions: At the within-person level in early adolescents, internalizing and externalizing problems demonstrate long-term bidirectional predictive effects and short-term unidirectional predictive effects; physical exercise exhibited both long-term protective and short-term buffering effects on these behavioral issues; both short-term and long-term physical exercise can mitigate the occurrence of externalizing problems by reducing internalizing issues; physical exercise may decrease internalizing problems through the reduction of externalizing behaviors over time, but this effect was not observed in the short-term.