Objective To explore the clinical value of the 5-factor modified frailty index (mFI-5) in predicting complications after total hip arthroplasty in the elderly with a view to providing a reference for patients with a comprehensive intervention during the peri-operative period. Methods A retrospective analysis was made of the clinical data of 639 patients aged ≥65 years, who underwent total hip arthroplasty in the Department of Orthopedics of the First Medical Center of the Chinese People′s Liberation Army General Hospital from January 2014 to August 2019. mFI-5 was calculated by collecting patient demographics, surgery-related information, mFI-5 scoring variables, and postoperative complications. According to the mFI-5 score, the patients were divided into a frail group (mFI-5≥2 points) and a non-frail group (mFI-5<2 points). Postoperative complications included postoperative delirium, stroke, pneumonia, deep vein thrombosis, and atelectasis. The two groups were compared in the incidence of postoperative complications. SPSS 26.0 was used for statistical analysis. Intergroup comparison was performed using t test or χ² test depending on data type. Multivariate logistic regression analysis was used to investigate the predictive effect of frailty on postoperative complications in patients with total hip arthroplasty. Results There were 190 cases (29.73%) in the frail group and 449 cases (70.27%) in the non-frail group. There were significant differences in gender, American Society of Anesthesiologists classification and anesthesia method between the two groups (all P<0.05). The frail group had significantly higher incidences in five aspects:history of chronic obstructive pulmonary disease or chronic pneumonia [27 (14.21%) vs 9 (2.00%)], dependent functional status [132 (69.47%) vs 43 (9.58%)], history of type 2 diabetes mellitus [92 (48.42%) vs 23 (5.12%)], history of congestive heart failure within 30 d before surgery [2 (1.05%) vs 0 (0.00)], and history of hypertension requiring drug treatment [176 (92.63%) vs 177 (39.42%)；all P<0.001] . The incidence of total complications in the frail group was significantly higher than that in the non-frail group [94 (49.47%) vs 20 (4.45%); P<0.001]. Multivariate logistic regression analysis showed that the risk of postoperative complications in the frail group was 12.23 times that of the non-frail group (OR=12.23,95%CI 6.51-23.98; P=0.002), and the difference was statistically significant (P<0.05). Frailty was independent risk factors of delirium (OR=10.32,95%CI 5.63-23.79; P=0.022), stroke (OR=12.24,95%CI 5.09-69.01; P<0.001), pulmonary infection (OR=5.88,95%CI 2.31-23.91; P<0.001) and DVT (OR=27.61,95%CI 3.02-78.24; P=0.034). Conclusion mFI-5 is an effective tool for predicting postoperative complications in the elderly patients with total hip arthroplasty, mFI-5≥2 being an important indicator to predict postoperative complications.