Objective To investigate the value of the change trend of lactic acid level 24 h after operation in predicting the complications of upper gastrointestinal ulcer perforation. Methods A total of 167 patients with upper digestive tract ulcer perforation who underwent surgical treatment in Shougang Hospital of Peking University from March 2021 to June 2023 were selected as the study objects, and were divided into mild to moderate group (n=117) and severe group (n=50) according to the severity of the disease. General data and lactic acid levels in 24 h after surgery were compared between the two groups. In addition, according to whether the patients had complications or not, they were divided into a group without complications (n=119) and a group with complications (n=48). The general data of the two groups were compared. A combined model was constructed, and Cox regression model was used to analyze the relationship between the change of lactic acid level at 24 h after operation and the complications of infection. Logistic regression model combined with restricted cubic spline model was used to analyze the dose-response relationship between lactic acid level and infection complications in patients with upper gastrointestinal ulcer perforation 24 h after operation. Log-binomial model was used to analyze the risk effect of complication types on lactic acid levels in patients with upper gastrointestinal ulcer perforation. Results At 8, 16 and 24 h after operation, lactic acid levels in both groups (mild to moderate group and severe group) were significantly lower than before operation, and lactic acid level in mild to moderate group was significantly lower than that in severe group (P<0.05). Repeated measurement ANOVA showed that the time effect, intergroup effect and interaction effect of lactic acid levels were significantly different between the two groups (P<0.05). There were significant differences in exhaust time, postoperative hospital stay, C-reactive protein, lactic acid level and red blood cell distribution width between the complicated group and the uncomplicated group (P<0.05). The combined model showed that each longitudinal increase of 1 mmol/L in lactic acid level 24 h after operation, was associated with a 4% increased risk of infection complications. The limit cubic spline map showed that lactic acid level at 24 h after operation was 4.22 mmol/L as the critical point of the dose-response relationship in the infection complications of patients with upper gastrointestinal ulcer perforation. Log-binomial model analysis results showed that when lactic acid level ≥4.22 mmol/L, patients with upper gastrointestinal ulcer perforation in the complication group had the highest risk of abdominal infection. The risk ratios (95%CI) before and after adjustment were 2.09 (1.25, 2.64), 2.16 (1.28, 2.05) and 2.20 (1.32, 2.63) times of those in the uncomplicated group, and the risk ratios of different lactic acid levels were statistically significant before and after adjustment (P<0.05). Conclusion Lactic acid level increased 24 h after operation in patients with upper gastrointestinal ulcer perforation is closely related to infection complications, and has a high predictive value for infection complications in patients with upper gastrointestinal ulcer perforation.