Objective: To understand the nutritional status and physical fitness of the Uygur, Kazak and Han ethnic college students, and to investigate the nonlinear relationship between BMI and physical fitness indexes in different ethnic groups. Methods: A total of 3 600 Uygur, Kazakh and Han students majoring in non-sports in a university in Xinjiang Uygur Autonomous Region in 2021 were selected by stratified random cluster sampling method. Height, weight, vital capacity, 50 m running, standing long jump, sit-and-reach, sit-up/pull-up and endurance running were measured. Body mass index (BMI), standardized Z score of each test score and physical fitness index (PFI) were calculated. The data were analyzed by Chi-square test, single factor analysis of variance, and nonlinear quadratic regression. Results: Prevalences of overweight (16.00%) and obesity (8.08%) of the Han college students were significantly higher than those of the Uygur (11.83% and 4.08%) and Kazakh (13.58% and 4.58%). Prevalence of low weight in the Uygur (11.92%) was the highest, and the lowest was the Kazakh (9.75%). There were significant differences in the prevalence of BMI classification among the three ethnic groups boys and girls (all P < 0.05). There were significant differences in PFI among college students of different BMI levels in the Uygur, Kazakh and Han ethnic college students (all P < 0.05), and the PFI of normal weight group was higher than the other weight groups in general, and the overweight group was higher than the obese group, but some ethnics showed the highest PFI in the low weight group. The non-linear quadratic regression results showed that the curves of the Uygur boys and girls and the Kazakh boys were inverted "J" shaped, and the PFI increased and then decreased with the increase of BMI, while the rest of the curves were arc-shaped, and the PFI decreased with the increase of BMI. Conclusion: Overweight obesity in the Uygur, Kazakh and Han ethnic college students brings about a decrease in physical fitness, but Kazakh low weight male and female and Han low weight grils have better physical fitness than the normal weight groups. Focusing on improving the physical fitness of Uygur low-weight and high-weight boys and Han high-weight girls could effectively reduce the differences in physical fitness of college students in different weight levels and ethnic groups.