Objective: To investigate the clinical characteristics, treatment and prognosis of gastric cancer complicated with immediate double primary cancer. Methods: The clinical data of patients who met the diagnostic criteria of gastric cancer with immediate double primary cancer from January 2016 to June 2019 were analyzed retrospectively. Results: There were 29 cases of gastric cancer with immediate double primary carcinoma, accounting for 1.7% of the 1 741 patients with gastric cancer in the same period. Of these, 17 (58.6%) were more than 70 years of age. 25 cases (86.2%) were male. The postoperative pathological staging was mainly for early gastric cancer (63.2%). Colorectal cancer accounted for 8 cases (27.6%), followed by esophageal cancer in 7 cases (24.1%). The others included 6 cases of lung cancer (20.7%), 2 cases of pancreatic cancer (6.9%), 2 cases of prostate cancer (6.9%), 1 case of non-Hodgkin's lymphoma (3.4%), 1 case of ampullary tumor (3.4%), 1 case of bile duct carcinoma (3.4%) and 1 case of laryngeal carcinoma (3.4%). The prognosis of the surgical treatment group was significantly better than that of the non-surgical treatment group (median survival time: 21.0 months vs 13.0 months, P=0.014). Conclusion: Gastric cancer complicated with immediate double primary cancer mostly occurs in elderly men and early gastric cancer patients. In the complicated tumor, colorectal cancer is the main cause, so we should pay attention to the screening of digestive system. Radical surgery should be performed as far as possible for each primary tumor.