Objective: To explore the associations of sedentary behavior and physical activity with next day affective states in junior and senior high school students, so as to provide a reference for promoting affective health in the population. Methods: From March to April 2026, 137 junior and senior high school students aged 12-17 years were recruited from two junior and senior high schools in Tianjin using convenience sampling. The ActiGraph wGT3X-BT accelerometer was used to continuously monitor 24 hour activity behaviors, and screen sedentary time (SST) and non-screen sedentary time were distinguished in combination with daily time use diaries. Daily affect levels in junior and senior high school students were collected via the Positive and Negative Affect Schedule-Expanded Form, with moods classified into high /low arousal positive affect and high /low arousal negative affect. Bayesian multilevel compositional data analysis and isotemporal substitution analysis were employed to explore the associations and substitution effects of sedentary behavior and 24 hour activity behaviors with next day affective states in junior and senior high school students. Results: Bayesian multilevel modeling showed that the associations between 24 hour movement behaviors and nextday affect in junior and senior high school students were mainly evident at the within person level. SST was associated with lower next day high arousal positive affect ( β =-0.15) and low arousal positive affect ( β = -0.11 ), and with higher high arousal negative affect ( β =0.12); light physical activity (LPA) was associated with higher high arousal positive affect ( β =0.13), and moderate to vigorous physical activity (MVPA) was associated with lower low arousal negative affect ( β =-0.06) (all P <0.05). Dose response analysis showed that, at the within person level, the substitution effect between SST and LPA was the most pronounced. As the duration of reallocating SST to LPA increased from 10 min to 60 min, high arousal positive affect ( β =0.020-0.123) and low arousal positive affect ( β =0.010-0.061) showed upward trends, while high arousal negative affect ( β =-0.061 to -0.010) showed a downward trend (all P <0.05); although low arousal negative affect presented a decreasing trend, it did not reach statistical significance at any time point ( β =-0.041 to -0.007, all P >0.05). Among the other substitution relationships, reallocating SST to MVPA was accompanied by a decrease in low arousal negative affect ( β =-0.033 to -0.005 ), and reallocating SST to sleep was accompanied by a decrease in high arousal negative affect ( β =-0.029 to -0.005) (all P < 0.05). Conclusions The associations between 24 hour activity behaviors and next day affect in junior and senior high school students are mainly evident at the within person level. Reallocating an individual s daily SST to LPA or MVPA can improve next day affective states in junior and senior high school students.