Objective: To explore the isotemporal substitution effects among different intensities of physical activity within a 10 minute recess period on the mental health of junior high school students, aiming to provide evidence based references for targeted practical interventions. Methods: From May to November 2024, a total of 845 junior high school students from Tianjin,Taiyuan and L Liang in Shaanxi Province,Puyang in Henan Province,Xi an in Shaanxi Province,Quzhou in Zhejiang Province,and Chaoyang in Liaoning Province were selected by using a combination of stratified random sampling and convenience sampling. ActiGraph wGT3X-BT accelerometers was used to measure physical activity during a 10 minute recess period. Mental health status was assessed with the Depression Anxiety Stress Scale (DASS-21). An isotemporal substitution model was constructed in 1 minute increments to predict the effects of substituting different physical activity behaviors on students mental health. Results: During recess, sedentary behavior (SB) was predominant among junior high school students, with an average duration of [7.08(5.85,7.98)] minutes, while moderate to vigorous physical activity (MVPA) accounted for the shortest duration at [0.42(0.21,0.85)] minutes. There were statistically significant differences in MVPA,LPA and SB time between students of different genders and grades( Z/H =-9.08,-8.34,-9.51;84.87,126.82,135.27,all P <0.01). Isotemporal substitution analysis, adjusted for gender and age, showed that replacing 1 minute of SB with 1 minute of MVPA significantly improved anxiety levels ( β =-0.29, 95% CI =-0.53 to -0.04) and overall mental health ( β =-0.72, 95% CI =-1.39 to -0.04), with both results reaching statistical significance (both P <0.05). No significant effects were observed for other substitution patterns (both P >0.05). Conclusions Substituting SB with MVPA during a 10 minute recess period exerts a positive impact on the mental health of junior high school students. It is recommended to optimize the daily recess activity structure in schools to enhance students mental well being.