Objective To explore the influence of the formation of gastric cancer tumor cells and immune cells cell-in-cell on the biological behavior of tumor cells. Methods The co-culture system of gastric cancer cells and mononuclear lymphocytes (PBMC) were divided into control group, 1∶1 group and 5:1 group. PBMC and BGC823 cells were seeded into 24-well plates at 1∶1 and 5∶1, respectively (5×10 4 cells in total). The construction of "cell-in-cell" was detected by Giemsa staining. Cell proliferation, apoptosis and invasion ability of each group were tested. Results The construction rate of "cell-in-cell" for the 1∶1 group was (8.65±1.06)%. The "cell-in-cell" construction rate of the 5∶1 group was (12.92±1.32)%, which was significantly higher than that of the 1∶1 group (P＜0.05). The biological behavior levels of the three groups of cells were significantly different (P＜0.05). The proliferation and invasion abilities of the 1∶1 group were significantly higher than those of the control group, while the apoptosis rate was significantly lower than that of the control group (P＜0.05). The proliferation and invasion abilities of the 5∶1 group were significantly higher than those of the 1∶1 group and the control group, while the apoptosis rate was significantly lower than that of the 1∶1 group and the control group (P＜0.05). Conclusion The "cell-in-cell" structure formed by gastric cancer cells and immune cells will promote the proliferation and invasion of gastric cancer cells and inhibit apoptosis.