Objective To observe the effects of postoperative complications, anal exhaust time, immune function and fatigue index of parenteral and enteral nutrition support after abdominal surgery in elderly patients with malnutrition diagnosed by GLIM criteria. Methods Ninty elderly abdominal surgery patients (more than 70 years old) who diagnosed as malnutrition by GLIM 2-step method were divided into enteral nutrition(EN) group and parenteral nutrition (PN) group randomly. The corresponding immune and fatigue indexes were tested on day 1 and day 7 of postoperation. Postoperative complications, anal exhaust time and mean time of hospitalization were observed and analyzed. Results The incidence of malnutritional risk was 61.0% (122/200) in elderly abdominal surgery patients and malnutrition incidence was 45.0% (90/200). IgA, IgM, IgG, CD3, CD4 and CD4/CD8 on postoperative day 7 increased significantly in EN group than those in PN group (P<0.05). The postoperative complications in EN group significantly lower than that in PN group (P<0.05). Anus exhaust time and hospitalization days of EN group were shorter than that of PN group (P<0.05). Fatigue index on postoperative day 7 of EN group was significantly better than that of PN group (P<0.05). Conclusions The malnutrition diagnosis of elderly patients based on GLIM crteria is a necessary prerequisite for nutritional intervention. Early EN support can reduce postoperative complications and hospitalization days in elderly patients, improve postoperative fatigue syndrome and immune status, and improve the patients with malnutrition recover from surgery.