Objective To investigate the effects of graded nutrition therapy guided by prognostic nutrition index PNI on immune function nutritional status and complications in patients with colorectal cancer. Method A total of 155 patients with colorectal cancer who were diagnosed and treated in Jiangsu Provincial People 's Hospital from November 2023 to August 2024 were selected as the research subjects and divided into the observation group treated with graded nutritional therapy under the guidance of PNI n = 78 and the control group treated with conventional nutritional intervention n = 77 according to the random number table method. The time of the first postoperative exhaust the time of eating the time of getting out of bed and the hospital stay were compared between the two groups. Observe the levels of PNI at admission T0 1 day before surgery T1 7 days after surgery T2 and 30 days after surgery T3 as well as the levels of serum CD4 + T cells CD4 + / CD8 + T cells CD8 + T cells albumin hemoglobin prealbumin at T0 and T3 and the complications. The correlation between PNI and complications was analyzed by Spearman method. Result The time of first exhaust after surgery the time of first eating after surgery the time of first getting out of bed after surgery and the hospital stay in the observation group were all shorter than those in the control group and the incidence of complications was lower than that in the control group P<0. 05 . The serum albumin hemoglobin and prealbumin of the two groups 30 days after the operation increased significantly compared with those at admission P < 0. 05 . The serum albumin hemoglobin and prealbumin in the observation group 30 days after the operation were all higher than those in the control group P<0. 05 . The CD4 + T cells CD4 + / CD8 + T cells and CD8 + T cells in the observation group at time point T3 were all higher than those in the control group P<0. 05 . The PNI of the observation group at time points T1 T2 and T3 was all higher than that of the control group P<0. 05 . Conclusion PNI guided fractional nutrition therapy is beneficial to improve nutritional status and immune function promote intestinal function recovery and reduce the incidence of complications in patients with colorectal cancer which is of great significance to improve the quality of life of patients.