Objective: To investigate the effects of after school exercise service (referred to as the "after school ES") on physical health, so as to provide evidence for possible beneficial effect of after school ES. Methods: Students in the fourth grade of primary school were recruited from September 2021 to July 2022 in Changsha City and divided into the control group ( n =220) and the after school ES group ( n =220). The control group only participated in the regular physical education activities of the school. The after school ES group received after school ES for one academic year, 4 times a week, 40-50 minutes per time, for a total of 32 weeks. Body shape indicators such as height, weight and percentage of body fat, as well as physical fitness indicators such as 50 meter running, grip strength and progressive aerobic cardiovascular endurance run (PACER) were measured in September to October 2021 and June to July 2022, respectively. Independent sample t-test, Chi square test and two factors repeated measurement analysis of variance were used for statistical analysis of the data. Results: After one academic year, compared with the control group [(13.52±2.30)kg], muscle mass of primary school students in the after school ES group [(13.76±2.32)kg] significantly increased, while waist to hip ratio [(0.95±0.16)] and percentage of body fat [(20.17±7.43)%] significantly decreased compared to the control group [(1.01±0.21), (22.02±12.34)%]( F=330.70, 6.85, 4.33, P <0.05). The proportion of overweight and obesity in after school ES group decreased significantly from 19.5% to 12.3% ( χ 2=4.35, P <0.05). Compared with the control group, the scores of 50 meter running [(10.00±1.06, 10.21±0.83)s], 1 minute sit up [(33.25±8.24, 30.76±9.34)times], sitting and flexion [(14.53±7.50, 8.59±6.32)cm], 1 minute rope skipping [(125.01±30.50, 115.97±32.09)times], eyes closed and single legged standing [(30.00±34.72, 25.72±23.82)s], selective response time [(635.66±91.72, 652.79±120.42)ms] and VO 2max [(45.31± 1.02 , 43.67±0.85)mL/(kg〖 ·min)] in the after school ES group were significantly improved, with statistical significance ( F= 5.32 , 443.14, 97.23, 814.07, 36.49, 6.11, 396.91, P <0.05). Conclusions After school ES can improve body shape of primary school students, reduce the risk of overweight and obesity and enhance physical fitness. It is recommended that schools should appropriately increase after school ES to promote physical fitness of students.