Objective: To understand current state of physical health levels of first year students in different body mass index (BMI) categories in Ningxia universities, and to explore the correlation between BMI and physical fitness index (PFI), so as to provide a reference for enhancing physical health levels of university students. Methods: In November 2024, physical fitness test data from 16 631 first year students across four universities in Yinchuan City, Ningxia from 2019 to 2023 were collected by adopting convenience and stratified cluster random sampling methods. The PFI was calculated using the Z score of the physical fitness test results, and a nonlinear quadratic model was established via least squares regression to examine the relationship between BMI and PFI among university students. Results: The BMI for males was (21.69±3.53)kg/m 2, while for females was (20.78±2.94)kg/m 2. The composite score for males physical fitness (69.86±9.25) was lower than that for females (72.24± 8.15 ), with a statistically significant difference ( t =-17.54, P <0.01). Moreover, the failure rates of various physical fitness indicators (vital capacity, sit and reach, standing long jump, pull ups/1 minute sit ups, 1 000 m/800 m run) were higher among males than females ( χ 2=103.48, 72.45, 14.38, 5 134.85, 188.89, all P <0.01). Comparisons across BMI categories revealed that among males, the normal weight group outperformed other groups in the 50 m sprint, standing long jump, 1 000 m sprint, composite score, and PFI ( F =89.17, 113.90, 179.02, 573.35, 593.08); among female students, the normal weight group outperformed other groups in the 50 m sprint, sit and reach, 800 m run, composite score, and PFI ( F =10.67, 19.58 , 96.45, 294.05, 183.45) (all P <0.01). The relationship between BMI and PFI among first year students exhibited a parabolic change trend, students with a moderate BMI demonstrated higher PFI, and as BMI increased, PFI decreased (all P <0.01). Conclusions The physical health level of male students in Ningxia universities is lower than that of female students. There is a correlation between BMI classification and PFI. Tailored intervention measures should be implemented according to the physical characteristics of students across different genders and BMI classifications to enhance university students physical health.