Objective To study the relationship of serum regulatory T cell (Treg) ratio, interleukin-17 (IL-17) and tumor marker carbohydrate antigen 72-4 (CA72-4) with therapeutic efficacy of immune checkpoint inhibitors (ICIs) for advanced gastric cancer (GC). Methods 96 patients with advanced GC who were treated with ICIs in the hospital from January 2022 to January 2024 were selected as study subjects, 96 patients were classified into treatment response group (0 cases of complete remission, 31 cases of partial remission, n=31) and treatment non-response group (40 cases of stable disease, 25 cases of disease progression, n=65), and 33 patients with early GC during the same period and 52 healthy physical examination subjects were collected. All patients received relevant laboratory examinations. The levels of serum Treg ratio, IL-17 and CA72-4 before treatment were collected. Receiver operating characteristic (ROC) curves of Treg ratio, IL-17 and CA72-4 in diagnosing the advanced GC and predicting the efficacy of ICIs were drawn. According to the cut-off values, the patients were divided into high Treg group, low Treg group, high IL-17 group, low IL-17 group, high CA72-4 group and low CA72-4 group Kaplan-Meier was used to draw the survival curves of different groups. Results Treg ratio, IL-17 and CA72-4 were manifested as treatment non-response group＞treatment response group＞early gastric cancer group＞healthy control group. ROC curve results revealed that the area under the curve (AUC) of combination of Treg ratio, IL-17 and CA72-4 in the diagnosis of advanced GC was the largest (0.969), and the AUC of the combination of the three indicators in predicting the efficacy of ICIs was 0.874. Kaplan-Meier analysis indicated that the mPFS values in high Treg group, high IL-17 group and high CA72-4 group were 7.9 months, 7.9 months and 7.1 months, which were shorter than 10.6 months, 10.9 months and 11.0 months in low Treg group, low IL-17 group and low CA72-4 group (Log Rank χ2=4.215, 4.996, 8.756, P=0.040, 0.025, 0.003). Conclusion The combination of Treg ratio, IL-17 and CA72-4 levels before treatment is of high value in the diagnosis of advanced GC and the prediction of ICIs efficacy. Monitoring the above indicators is conducive to providing guidance for the ICIs treatment of advanced GC