Objective This study aims to investigate the incidence of postoperative delirium (POD) and related risk factors in elderly patients with thoracic or abdominal malignant tumor. Methods A total of 246 patients underwent malignant tumor surgery at the Xuzhou Medical University Affiliated Hospital from January to August 2019 were enrolled. The patients' information was collected according to preoperative and postoperative interview. This study mainly observed the incidence of POD, and used multivariate Logistic regression to analyze the independent risk factors of POD in elderly patients with malignant tumors. Results A total of 77 (31.3%) patients were postoperative delirium. Logistic regression analysis showed advanced age [odd ratio (OR)=1.096, 95% confidence interval (CI) 1.038‒1.158], postoperative intensive care unit (ICU) admission (OR=3.246, 95%CI 1.587‒6.641), low preoperative Mini‐Mental State Examination (MMSE) score (OR=0.905, 95%CI 0.835‒0.982), low serum albumin level (OR=0.929, 95%CI 0.871‒0.990) and prolonged anesthesia (OR=1.004, 95%CI 1.000‒1.008) were independent risk factors for POD in elderly patients with malignant tumor. There was no statistically significant difference in the length of surgery (OR=0.997, 95%CI 0.981‒1.012), intraoperative bleeding (OR=1.000, 95%CI 0.999‒1.001) and the length of time from surgery to discharge (OR=0.993, 95%CI 0.932‒1.059) between the two groups (P>0.05). Conclusions Advanced age, postoperative ICU admission, low preoperative MMSE score, low serum albumin levels and prolonged anesthesia were risk factors for POD in elderly patients with malignancies.