The effect of intra-operative chemotherapy (IOC) on the long-term survival of patients with colorectal cancer (CRC) remains unclear. In this study, we evaluated the independent effect of intra-operative infusion of 5-fluorouracil in combination with calcium folinate on the survival of CRC patients following radical resection.1820 patients were recruited, and 1263 received IOC and 557 did not. Clinical and demographic data were collected, including overall survival (OS), clinicopathological features, and treatment strategies. Risk factors for IOC-related deaths were identified using multivariate Cox proportional hazards models. A regression model was developed to analyze the independent effects of IOC.Proportional hazard regression analysis showed that IOC (hazard ratio [HR]=0.53, 95% confidence intervals [CI] [0.43, 0.65], P  < 0.001) was a protective factor for the survival of patients. The mean overall survival time in IOC group was 82.50 (95% CI [80.52, 84.49]) months, and 71.21 (95% CI [67.92, 74.50]) months in non-IOC group. The OS in IOC-treated patients were significantly higher than non-IOC-treated patients ( P  < 0.001, log-rank test). Further analysis revealed that IOC decreased the risk of death in patients with CRC in a non-adjusted model (HR=0.53, 95% CI [0.43, 0.65], P  < 0.001), model 2 (adjusted for age and gender, HR=0.52, 95% CI [0.43, 0.64], P  < 0.001), and model 3 (adjusted for all factors, 95% CI 0.71 [0.55, 0.90], P  = 0.006). The subgroup analysis showed that the HR for the effect of IOC on survival was lower in patients with stage II (HR = 0.46, 95% CI [0.31, 0.67]) or III disease (HR=0.59, 95% CI [0.45, 0.76]), regardless of pre-operative radiotherapy (HR=0.55, 95% CI [0.45, 0.68]) or pre-operative chemotherapy (HR=0.54, 95% CI [0.44, 0.66]).IOC is an independent factor that influences the survival of CRC patients. It improved the OS of patients with stages II and III CRC after radical surgery.chictr.org.cn, ChiCTR 2100043775.Copyright © 2023 The Chinese Medical Association, produced by Wolters Kluwer, Inc. under the CC-BY-NC-ND license.