BackgroundTo investigate the effect of body mass index (BMI) on clinical outcomes after robotic cardiac surgery, and to explore the postoperative obesity paradox.MethodsThe data of 146 patients who underwent robotic cardiac surgery under cardiopulmonary bypass (CPB) from July 2016 to June 2022 in Daping Hospital of Army Medical University were retrospectively analyzed, and their demographic data and related clinical data were statistically analyzed. The mean age was (42.88 ± 13.01) years, 55 (37.67%) were male and 91 (62.33%) were female. Patients were divided into 3 groups according to preoperative BMI: lean group (BMI < 18.5 kg/m2;n= 17; 11.64%), normal group (BMI 18.5 kg/m2to 23.9 kg/m2;n= 81; 55.48%), and overweight and obese group (BMI ≥ 24 kg/m2;n= 48; 32.88%). Multivariate analysis was performed to compare clinical outcomes across BMI groups.ResultsPreoperative data in different BMI groups showed that there were statistically significant differences in age, height, weight, body surface area (BSA), diabetes, left atrial anteroposterior diameter (LAD), triglyceride (TG), and high-density lipoprotein (HDL) (allP< 0.05). Postoperative clinical outcomes showed that there was no statistical difference between the lean group and the normal group; the intensive care unit stay and postoperative hospital stay in the overweight and obese group were significantly higher than those in the normal group (P< 0.05), and the risk of postoperative cardiac surgery-related acute kidney injury (CSA-AKI) was significantly increased (P= 0.021); further Multiple Binary Logistic Regression Analysis suggested that preoperative TG (OR= 1.772, 95%CI1.068–2.942,P= 0.027) and operation time ≥ 300 min (OR= 3.823, 95%CI1.098–13.308,P= 0.035) were independent risk factors for postoperative CSA-AKI.ConclusionsOverweight and obese patients had significantly prolonged intensive care unit stay and postoperative hospital stay after robotic cardiac surgery, and significantly increased incidence of postoperative CSA-AKI, which did not support the obesity paradox; preoperative TG and operation time ≥ 300 min were independent risk factors for postoperative CSA-AKI.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12872-023-03277-w.