Objective To investigate the relationship between preoperative body mass index (BMI) and the risk of postoperative complications in elderly patients. Methods This was a secondary analysis on the data derived from the patients of the placebo group in a previously published randomized controlled trial. A total of 350 elderly patients (≥65 years old) who were admitted to intensive care unit (ICU) after noncardiac surgery were recruited in this study. The primary endpoint was the occurrence of postoperative complications. Logistic regression model was used to analyze the relationship between preoperative BMI grade and the risk of postoperative complications.Results There were 35.1% of patients (123/350) developing postoperative complications. Logistic multivariate regression analysis showed that, compared with the normal weight patients (BMI 18.5-23.9 kg/m²), the underweight (BMI <18.5 kg/m²) was associated with an increased risk of postoperative complications (OR=2.210,5%CI:1.069-4.570, P=0.032), whereas overweightness and obesity (BMI ≥24.0 kg/m²) had no such significant effect (OR=0.820,5%CI:0.497-1.354, P=0.438). Conclusion For the elderly who were admitted to ICU after noncardiac surgery under general anesthesia, low preoperative BMI is associated with increased risk of postoperative complications.