Objective: To measure the body mass index (BMI) of secondary school students and analyze its association with the physical fitness index, so as to improve the physical health of middle school students. Methods: Using a randomized cluster stratified sampling method, 3 813 secondary school students from six cities (Ganzhou, Ji an, Fuzhou, Nanchang, Shangrao and Jiujiang) in Jiangxi Province, China, were selected in September 2022. Questionnaire survey was administered to obtain basic information and to assess the nutritional status and physical fitness tests were conducted among secondary school students. The study was stratified by gender and age, and the Kruskal Wallis H test was used to compare the differences in the mean values of grip strength, sit-ups, standing long jump performance and strength quality among secondary school students with different BMI levels, while the correlation between BMI and strength quality index was inferred by using curve regression analysis. Results: There was statistical significance in the strength quality index of middle school students with different BMI level ( H=56.79, 8.84, P <0.05). Except 13 years old group, the difference of physical fitness by BMI grade strength quality index of boys in other age groups was statistically significant ( H =22.21, 16.23, 17.98, 18.21, 8.08), while the difference of physical fitness by BMI of girls at 14 years old was statistically significant ( H =9.11)( P <0.05). The curve fitting using regression showed that the physical fitness index and BMI of middle school students (boys and girls) show an inverted U-shaped curve. The physical fitness index was higher in the middle region of BMI ( Z-score) and lower in the two end regions, while showing a decreasing trend from the middle to the two ends. Conclusion An association is found between BMI and physical fitness among secondary school students in Jiangxi Province, and wasting and overweight/obesity are negatively correlated with physical fitness. Greater attention should be paid to the nutritional status of secondary school students to ensure the normal development of physical fitness.