Abstract:As the compositional nature of "constant sum" of physical activity, sedentary behavior and sleep are widely recognized, the methodological and result limitations of previous studies that considered physical activity, sedentary behavior, or sleep as independent influences on physical fitness and health have been highlighted. At present,physical activity guidelines of various countries have shifted from local recommendations based on physical activity amount to global recommendations based on 24 h time frame. From the perspective of time use, the content of daily schedule of children and adolescents also has the attribute of "constant sum" of compositional data, and the reduction of students homework burden and extracurricular discipline training after "double reduction" inevitably means an increase in the time allocated to other activities. From the perspective of time use, the paper focuses on physical fitness and health promotion strategies in the context of "double reduction".