Abstract: Objective To investigate the expression level and diagnostic value of serum cysteine protease inhibitor S (CST4) in patients with gastric cancer. Methods Clinical data of 115 patients with suspected gastric cancer who complained of gastric discomfort were retrospectively analyzed, and they were divided into benign disease group (n=50),precancerous disease group (n=26) and gastric cancer group (n=39). The levels of serum CST4, carcinoembryonic antigen (CEA), carbohydrate antigen 19-9 (CA19-9) and carbohydrate antigen 72-4 (CA72-4) were analyzed in the three groups. The positive rates of CST4 among the three groups were compared. Binary Logistic regression analysis was used to screen for independent risk factors for gastric cancer occurrence. The receiver operating characteristic (ROC) curve was used to evaluate the diagnostic value of CST4 in gastric cancer. Results The positive rate of CST4 was 6.00% (3/50) in the benign gastric disease group, 30.77% (8/26) in the gastric precancerous lesion group, and 66.67% (26/39) in the gastric cancer group. The positive rate of CST4 in the gastric cancer group was higher than that in the gastric precancerous lesion group and the benign gastric disease group(P<0.05). The results of binary Logistic regression analysis showed that advanced age, high levels of serum CST4 and high levels of CEA were independent risk factors for gastric cancer occurrence (P<0.05). The area under the curve (AUC) for CST4 alone in diagnosing gastric cancer was 0.847 (95%CI, 0.760 to 0.934), with an optimal cutoff value of 94.6 U/mL, the Youden index of 0.638, sensitivity of 71.8%, and specificity of 92.0%. The AUC for the combined diagnosis of gastric cancer using CST4, age and CEA was 0.959 (95%CI, 0.919 to 0.992), with sensitivity of 94.9% and specificity of 86.0%. Conclusion As a novel serum marker, CST4 has high predictive value in the auxiliary diagnosis of gastric cancer.