Objective?To observe changes of immune function in patients with colorectal cancer after perioperative allogeneic transfusion and the effects of allogeneic transfusion on prognosis. Methods?A total of 115 patients with colorectal cancer who underwent surgical treatment in The Sixth Affiliated Hospital, Sun Yat-sen University from October 2016 to August 2020 were selected as the research subjects for retrospective analysis, and were divided into transfusion group (66 cases) and non-transfusion group (49 cases) according to whether they received perioperative blood transfusion. The transfusion group was given perioperative allogeneic transfusion with red blood cells or plasma. Before and after transfusion, levels of T cell subsets (CD3+ , CD4+ , CD8+ , CD4+ /CD8+ ) and coagulation function were detected. The changes of tumor related markers before and after surgery were compared, and the overall survival rate of the two groups was compared. Results?After transfusion, the percentage of CD3+ , CD4+ and CD4+/ CD8+ in the transfusion group was significantly lower than that before transfusion, and the percentage of CD8+ was significantly higher than that before transfusion (P ＜ 0.05). The percentage of CD3+ , CD4+ and CD4+ /CD8+ in the transfusion group were lower than those in the non-transfusion group, while the percentage of CD8+ was higher than that in the non-transfusion group (P ＜ 0.05). There was no significant difference in coagulation function between two groups before and after transfusion or fluid infusion (P ＞0.05). There was no significant difference in the positive expression rates of tumor related markers between two groups before and after surgery (P ＞ 0.05). Follow-up of survival data up to January 2021, the overall survival rate of the transfused group was 81.82%, and that of the nontransfused group was 79.59%, with no statistically significant difference between the two groups (HR=0.965, P=0.934, 95%CI= 0.413~2.253). Conclusion?Perioperative allogeneic transfusion can affect T lymphocyte subsets and impair immune function. Clinicians should comprehensively assess immune status of patients and determine allogeneic transfusion to reduce damage to immune mechanisms.