Objective To explore the potential relationship between preoperative frailty condition and postoperative adverse outcome in elderly patients undergoing brain tumor resection, in order to provide effective evidence for comprehensive perioperative intervention in such patients. Methods A total of 553 patients aged 65 years and older who underwent brain tumor resection at the First Medical Center of PLA General Hospital from January 8, 2014 to August 8, 2019 were enrolled and their clinical data were retrospectively analyzed. Their preoperative frailty was assessed by the 5‑factor modified weakness index (mFI‑5). The general data, mFI‑5 score variables and postoperative complications were collected to construct the mFI‑5 score. Based on the mFI‑5 score, the patients were divided into two groups: a frail group (n=157, mFI‑5≥0.4 score) and a non‑frail group (n=396, mFI‑5<0.4 score). The primary outcome was one‑year all‑cause mortality, and the secondary outcomes were pulmonary infection and stroke. The one‑year all‑cause death was analyzed by COX regression, and stroke and pulmonary infection were analyzed by logistic regression. Kaplan‑Meier survival curves were plotted and Log‑Rank was used to compare the survival curves of the two groups. Results The frail group showed remarkably higher percentages of postoperative one‑year all‑cause death, pulmonary infection, and stroke than the non‑frail group (P<0.05). The probability of one‑year all‑cause death in the frail group was 3.987 times as high as that in the non‑frail group ［odds ratio (OR) 3.987 (95%CI 1.542, 5.978), P=0.011］. The Kaplan‑Meier curves showed poorer prognosis in the frail group than those in the non‑frail group (P<0.05). The frail group presented a probability stroke which was 4.830 times as high as the non‑frail group ［OR 4.830 (95%CI 2.847, 8.196), P<0.001］ and a probability of pulmonary infection which was 4.911 times as high as the non‑frail group ［OR 4.911 (95%CI 1.921, 7.214), P=0.003］. Conclusions Frailty is an independent risk factor for poor postoperative outcome in elderly patients after brain tumor resection. The mFI‑5≥0.4 score can be used as an effective indicator to identify frailty and provide effective evidence for postoperative comprehensive intervention.