Abstract Objective: To investigate the relationship between Body Mass Index (BMI) and Physical Fitness Index (PFI) among middle and high school students aged 13–18 in Shanxi Province, and to provide a reference for adolescent physical health promotion. Methods: A total of 3,600 middle and high school students aged 13–18 (1,800 boys and 1,800 girls) from Shanxi Province who participated in the 2025 National Physical Fitness and Health Survey were selected as the study subjects.