Objective: To investigate the correlation between fat mass percentage(FM%), fat mass index(FMI), fat free mass index(FFMI), body mass index (BMI) and physical fitness, and to explore the potential value of body composition index in predicting physical exercise level of children and adolescents. Methods: Data came from 2013 revision of National Students Physical Health Standard of Ministry of Education in Beijing. Body composition of 4 069 children and adolescents aged 6-20 years in Beijing was measured by the BIA(inbody 230). FM, FMI, FFMI and BMI were divided into four groups by quartile method. The differences and trends of the four groups were analyzed by graphpad prism 8.0 software. Results: FM% (20.03±10.39) and FMIS [(4.35± 2.84 )kg/m 2] of oys were significantly lower than those of girls, while FFMI [(14.21±4.95)kg/m 2] and BMI [(20.31±4.27) kg/m 2 ] of boys were significantly higher than those of girls( t =-13.36, -7.66, 11.49, 8.16, P <0.01). In boys, with the increase of FM% and FMI, the performance of 50 m, 1 000 m, standing long jump and pull up showed a downward trend; with the increase of FFMI, the performance of 50 m, standing long jump and pull up had an obvious upward trend. In girls, with the increase of FM% and FMI, the 50 m running time was gradually shortened, and the 800 m running time was gradually increasing; High FFMI was associated with higher performance in 50 m running, while no association was observed between FFMI and 800 m running.Four body composition indicators showed no effects on performance of sitting forward flexion and 1 minute sit up. Conclusion FM%, FMI, FFMI and BMI have a good predictive effect on physical fitness of children and adolescents. Adolescents with higher FFMI, lower FM% and FMI have better muscle burst strength, muscle endurance, flexibility, aerobic ability and anaerobic ability, especially in boys.