Objective To explore the association between BMI and the risk of developing cardiac surgery associated acute kidney injury (CS-AKI),mortality of AKI and AKI requiring renal replacement therapy (AKI-RRT) after cardiac surgery.Methods Clinical data of patients undergoing cardiac surgery from January 2011 to December 2015 in Zhongshan Hospital of Fudan University were prospectively collected.Patients were divided into four groups according to BMI classification of Chinese population.Adjustment for selection bias was further assessed using propensity score method (PSM) to evaluate the role of BMI in the development of AKI.Results A total of 8442 patients were enrolled,among which 1092 patients successfully matched through PSM.The AKI incidences were respectively 30.3％,33.3％,38.6％ and 46.8％ in four BMI groups (P ＜ 0.01) before PSM.The AKI incidences were respectively 31.9％,35.2％,42.5％ and 42.9％ in four BMI groups (P=0.016) after PSM.The risk of developing AKI increased by 19.9％ as the BMI increased per 5 kg/m2 (95％ CI:1.070-1.344,P=0.002).The hospital mortality of patient (overall,AKI,AKI-RRT) in four groups was not statistically different after PSM (P ＞ 0.05),but overweight group always had the lowest mortality.Conclusions BMI is a risk factor for AKI after cardiac surgery,and the AKI incidence increases with increasing BMI in a certain range.