To examine the association between moderate-to-vigorous physical activity (MVPA) and weight-adjusted waist index (WWI) with psychological sub-health in middle school students, so as to provide a basis for mental health intervention and promotion.

From September to November 2023, a stratified cluster random sampling method was used to survey 47 372 middle school students aged 12-17 in Jiangsu, Xinjiang, Guangdong, Jilin and Henan provinces from eastern, western, southern, northern and central of China. MVPA, WWI, and psychological subhealth were assessed, with WWI categorized into quartiles (Q1-Q4). Data were analyzed using χ² test, Logistic regression analysis and Logistic regression analysis of generalized linear models.

The overall prevalence of psychological sub-health among middle school students was 21.0%, with higher rates in boys(21.4%) than girls(20.5%) (χ²=6.49, P < 0.05). The detection rates of emotional problems, behavioral problems and social adaptation difficulties in middle school students were 27.8%, 26.9% and 17.5%, respectively. The combination of MVPA and WWI groups was used as independent variables, and the covariates were adjusted to perform Logistic regression analysis of generalized linear model; while the Q1 group with MVPA>60 min/d was used as the reference group, the Q4 group with MVPA < 30 min/d had the highest risk of psychological sub-health detection(OR=1.58, 95%CI=1.38-1.81), and the detection risks of psychological sub-health among middle school students in Q1-Q3 groups were 1.34(1.17-1.53), 1.30(1.13-1.49) and 1.42(1.24-1.62)(P < 0.01).

MVPA and WWI are significantly associated with psychological sub-health in middle school students. Lower MVPA and higher WWI in middle school students correlate with increased sub-health risks. Interventions should aim to enhance MVPA levels and reduce WWI to improve mental well-being in middle school students.