Objective To evaluate the incidence and risk factors for organ/space surgical site infection (SSI) after gastric and colorectal cancer surgery, and construct simple and effective risk prediction models for patients. Methods Clinical data were retrospectively collected from 356 patients with gastric cancer and 305 patients with colorectal cancer who underwent radical surgery in a hospital from January 2018 to October 2022. Independent risk factors for organ/space SSI were identified by univariate and multivariate logistic regression analysis, and nomogram models were constructed. The concordance index (C-index) and calibration curve were used to evaluate the predictive performance of the gastric and colorectal cancer models. Results The incidences of postoperative organ/space SSI in gastric and colorectal cancer patients were 8.4% (30/356) and 9.5% (29/305), respectively. Patients with organ/space SSI had a longer hospital stay than those without SSI (P < 0.001). Multivariate analysis showed that male (OR=5.203, 95%CI: 1.167-23.193), pre-operative ASA grade Ⅲ(OR=5.078, 95%CI: 2.047-12.598), combined organ resection (OR=3.479, 95%CI: 1.261-9.546) and intraoperative blood loss (OR=3.629, 95%CI: 1.577-8.350) were independent risk factors for organ/space SSI after gastric cancer surgery (all P < 0.05). ASA grade (OR=3.128, 95%CI: 1.334-7.337), combined organ resection (OR=3.442, 95%CI: 1.292-9.168) and duration of operation (OR=3.091, 95%CI: 1.367-6.991) were independent risk factors for organ/space SSI after colorectal cancer surgery (all P < 0.05). Based on these risk factors, nomogram models for predicting organ/space SSI after gastric and colorectal cancer surgery were developed, with C-indexes of 0.808 (95%CI: 0.722-0.894) and 0.763 (95%CI: 0.674-0.852), respectively. The calibration curves indicated good consistency between the predicted curves and the ideal reference lines. Conclusion The nomogram models have an excellent prediction performance for organ/space SSI in gastric and colorectal cancer patients underwent radical resection, and provide a simple and effective tool for clinicians to identify high-risk individuals with SSI.