To investigate the relationship between body mass index (BMI) and early prognosis in patients with acute stroke in the Xi’an region. Methods: Clinical data of acute stroke patients who were consecutively admitted to 4 tertiary grade A hospitals in the Xi’an area from January 2015 to December 2015 were collected. The clinical characteristics of acute stroke patients with different ranges of BMI (classified as groups underweight, normal weight, overweight, and obese) were compared, and the outcome events (including stroke recurrence, death, and poor prognosis) were followed up for 90 days. Univariate and multivariate Logistic regression analysis was performed to investigate the relationship between BMI and early prognosis in these patients. Results: A total of 2 692 acute stroke patients were enrolled in this study, with a mean age of (63.58± 12.46) years. Follow-up at 90 d revealed significantly higher mortality rate in the low BMI group (12.12% for low vs. 3.70% for normal, 4.42% for overweight, and 2.88% for obese) with statistically significant differences (P<0.001), but there were no statistically significant differences between the groups in poor prognosis rate and stroke recurrence rate at 90 d (P=0.113 and P=0.495). After adjusting for relevant confounders, multivariate Logistic regression analysis showed that low BMI was an independent risk factor for 90 d mortality in acute stroke patient in the Xi’an area (OR=3.72, 95% CI 1.54~9.00, P=0.004) but was not an independent risk factor for stroke recurrence and poor prognosis at 90d (P>0.05). Conclusion: Low BMI was an independent risk factor for early mortality in acute stroke patient in the Xi’an region.