Objective: To investigate the effect of allogeneic red blood cell transfusion on T lymphocyte subsets and natural killer (NK) cells in peripheral blood of patients with gastric cancer during perioperative period. Methods: 50 patients with gastric cancer in our hospital from January 2017 to December 2017 were randomly divided into control group (n=25) and observation group (n=25). Perioperative allogeneic erythrocyte transfusion was performed in the observation group, while no allogeneic erythrocyte transfusion was performed in the control group. Blood samples were taken from two groups before and after operation 3 d, and serum T lymphocyte subsets (CD3⁺, CD4⁺, CD8⁺, CD4⁺ /CD8⁺ ) and NK cell levels were measured. T cell subsets and NK cell levels of patients in observation group, control group and observation group at different clinical stages before and after operation were counted. According to blood transfusion volume, the patients in the observation group were divided into <4 U group and <4 U group. T cell subsets and NK cell levels before and after operation were compared between the two groups. Results: There was no significant difference in CD3⁺, CD4⁺, CD8⁺, CD4⁺ /CD8⁺, NK cells between the two groups before operation (P>0.05). The levels of CD3⁺, CD4⁺, CD4⁺ /CD8⁺, NK cells in the two groups after operation were lower than those before operation and CD8⁺ levels were higher than those before operation. The levels of CD3⁺, CD4⁺, CD4⁺ /CD8⁺, NK cells in the observation group were lower than those in the control group, and the levels of CD8⁺ in the observation group were higher than those in the control group (P<0.05). The levels of CD3⁺, CD4⁺, CD8⁺, CD4⁺ /CD8⁺, NK cells in patients with stage Ⅰ-Ⅱ and stage Ⅲ-Ⅳ had no significant difference before operation (P>0.05). After operation, the levels of CD3⁺, CD4⁺, CD4⁺ /CD8⁺, NK cells in the two groups were lower and CD8⁺ levels were higher than those before operation. The levels of CD3⁺, CD4⁺, CD4⁺ /CD8⁺, NK cells in stage Ⅰ-Ⅱ patients were higher than those in stage Ⅲ-Ⅳ patients, and CD8⁺ levels were lower than those in stage Ⅲ-Ⅳ patients (P< 0.05); There was no significant difference in the levels of CD3⁺, CD4⁺, CD8⁺, CD4⁺ /CD8⁺, NK cells between the <4 U group and <4 U group before operation (P>0.05). After operation, the levels of CD8⁺ in the two groups were higher than those before operation, while the levels of CD3⁺, CD4⁺, CD4⁺ /CD8⁺, NK cells were lower than those before operation; The levels of CD3⁺, CD4⁺, CD4⁺ /CD8⁺, NK cells in the <4 U group were higher than those in >4 U group, and CD8⁺ levels were lower than those in >4 U group (P<0.05). Conclusions Perioperative allogeneic red blood cell transfusion can affect T cell subsets and NK cell content in peripheral blood of patients with gastric cancer, and there are some differences in the effects of different clinical stages of gastric cancer patients, which can cause the suppression of cellular immune function.