Objective: To investigate the effects of two different bowel preparation methods on postoperative surgical site infection (SSI) of gastrointestinal malignant tumors. Methods: A total of 238 patients with gastrointestinal malignancies were randomly divided into two groups preoperatively: mechanical bowel preparation MBP group and oral antibiotic bowel preparation (ABP) combined MBP group (combination group), and the difference in SSI incidence between the two groups was analyzed. Postoperatively, the patients were divided into SSI and non SSI groups according to the diagnostic criteria of SSI, and the levels of white blood cell (WBC), albumin (ALB), procalcitonin (PCT), and C-reactive protein (CRP) were monitored perioperatively to analyze the predictive effect of each blood indicator on SSI and to screen the risk factors for developing SSI. Results: The incidence of SSI in combination group was lower than that in MBP group (χ²=12.472, P=0.000). According to the results of repeated measurement variance, the overall difference of PCT level between the two groups was statistically significant (F=42.983, P=0.000), and the SSI group was significantly higher than the non-SSI group at the 1st, 3rd and 5th day after surgery. The overall difference of CRP level between the two groups was statistically significant (F=18.280, P=0.000), and the SSI group was significantly higher than the non-SSI group at 3 and 5 days after operation. In the ROC curve, the area under the PCT curve on the 5th day after operation was 0.942, and when the cut-off point was 1.075 μg/L, the sensitivity and specificity of SSI diagnosis were 0.917 and 0.878 respectively. Logistic regression analysis showed that there was a significant correlation between preoperative ABP, PCT level on the 5th day after surgery and postoperative SSI (P＜0.05), and PCT level on the 5th day after surgery was an independent risk factor for SSI , and preoperative oral antibiotics was an independent protective factor for SSI . Conclusions: MBP combined with ABP can significantly reduce the incidence of postoperative SSI in patients with gastrointestinal malignancies. PCT can predict the occurrence of SSI earlier, and play an early warning role in the occurrence of postoperative SSI, which is conducive to guiding the early clinical treatment of SSI.