Abstract The overall development of adolescents is important for achieving the long-range goals for 2035 and the great rejuvenation of the Chinese nation. Since "physical and mental health" is fundamental for individual comprehensive development, it is necessary to pay attention to the physical and mental health of adolescents to promote their overall development. As far as the current situation is concerned, the development of the physical and mental health of Chinese adolescents shows positive improvement but still faces many challenges. The outbreak and spread of COVID-19 has changed the lifestyle, learning habits and social interactions of adolescents to a great extent, exerting negative impacts on physical and mental development. In the context of the normalization of epidemic prevention and control, it is necessary to further investigate the association between physical fitness with mental health, and to explore the various factors influencing this relationship. It is of great value to understand the importance of physical and mental health promotion of adolescents under the new conditions, in the maintenance and improvement of overall youth development at present and in the future.

Objective: To understand the relationships between video time, exercise time and the mental sub-health of Chinese adolescents, and to assist the development of Chinese adolescents’ mental health. Methods: In this study, 16 545 adolescents aged 13-22 years in six administrative regions of China were surveyed using an adolescent sub-health multi-dimensional assessment questionnaire (MSQA), and daily physical exercise time, video screen time and other indicators were recorded. Binary Logistic regression analysis was used to understand adolescents’ mental sub-health and the correlation between video time and exercise time. Results: Detection rate of mental sub-health status in adolescents with video time ≤2 h/d was lower than that of adolescents with video time >2 h/d(19.1%，22.1%), and the detection rate of adolescents with exercise time ≤60 min/d(22.1%，17.7%) was higher than that of adolescents with exercise time >60 min/d. These differences were statistically significant (χ2=14.47, 6.97, P<0.05). Binary Logistic regression analysis showed that the risk of mental sub-health status for Chinese adolescents whose screen time was more than 2 h/d was 1.20 times that of those with screen time ≤2 h/d, and the difference was statistically significant (P<0.01). The risk of mental sub-health for students whose exercise time was > 60 min/d was 0.86 times that of students who exercised ≤ 60 min/d, and the difference was statistically significant (P<0.05). Conclusion Screen time >2 h/d and exercise time <60 min/d were negative factors leading to mental sub-health symptoms in Chinese adolescents.It is proposed to jointly promote the healthy adolescent development through health education,as well as positive family and social environment.