Objective To explore the correlation between body mass index (BMI) and disease severity in patients with spinocerebellar ataxia type 3 (SCA3). Methods Patients who visited the Department of Neurology of the First Affiliated Hospital of Fujian Medical University with a confirmed diagnosis of SCA3 between July 2022 and August 2023 were selected as a case group, and healthy individuals between June 2024 and October 2024 were selected as a control group, and the BMI levels of the two groups were compared. Patient demographics and clinical statistics were collected, the severity of ataxia in SCA3 patients was assessed using the Scale for the Assessment and Rating of Ataxi, and the relationship between BMI and disease severity was evaluated. Results A total of 101 patients and 101 healthy individuals were included. The BMI levels of SCA3 patients were significantly lower than those of normal controls (t=−2.370, P=0.019). The results of the multiple linear regression model showed that the BMI, disease duration and smoking history had an effect on the disease severity of SCA3 patients (P<0.05), and disease duration and disease severity had a significant effect on the disease progression in SCA3 patients (P<0.05). Conclusion There may be a correlation between BMI and disease severity in SCA3 patients, and controlling the BMI level may help to control the disease in SCA3 patients.