Background: Impact of body mass index (BMI) on all-cause mortality in atrial fibrillation (AF) patients remains controversial. Methods: A total of 10,942 AF patients were prospectively enrolled and categorized into four BMI groups: underweight (BMI < 18.5 kg/m 2 ), normal weight (BMI 18.5–24 kg/m 2 ), overweight (BMI 24–28 kg/m 2 ) and obesity (BMI ≥ 28 kg/m 2 ). The primary outcome was all-cause mortality. Different Cox proportional hazards models were performed to evaluate the association between BMI and all-cause mortality. Results: During a median follow-up of 30 months (IQR 18–48 months), 862 deaths events occurred. Compared to normal BMI, higher BMI was associated with a lower mortality risk (overweight: HR 0.70; 95% CI 0.61–0.81, P < 0.0001 and obesity: HR 0.54; 95% CI 0.44–0.67, P < 0.0001) and lower BMI was associated with a higher mortality risk (HR 2.23, 95% CI 1.67–2.97, P < 0.0001). Conclusion: A reversed relationship between BMI and all-cause mortality in AF patients was found. Higher risk of mortality was observed in underweight patients compared to patients with a normal BMI, while overweight and obese patients had a lower risk of all-cause mortality. Clinical trial registration: URL: http://www.chictr.org.cn/showproj.aspx?proj=5831. Unique identifier: ChiCTR-OCH-13003729. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.