Background To compare the predictive power of three different evaluation methods of frailty for 90-day outcomes of elderly patients undergoing elective abdominal surgery. Methods A prospective cohort study was conducted with 194 patients and a postoperative follow-up period of 90 days. Preoperative frailty was evaluated using the five-item FRAIL questionnaire, 54-item frailty index (FI), and nine-item Clinical Frailty Scale (CFS). Receiver operating curves were used to compare the predictive ability for 90-day mortality and long-term hospitalization (LTH), and logistic regression was used to calculate odds ratios and 95% confidence intervals. Results The incidence rates of frailty assessed using FRAIL, FI, and CFS criteria were 43.8%, 32.5% and 36.6%, respectively. The 90-day mortality and LTH of frail patients were significantly higher than those of non-frail patients regardless of which criteria were used. The CFS and FI predicted 90-day mortality better than FRAIL (CFS versus FRAIL:P= 0.005; FI versus FRAIL:P= 0.041), and the CFS predicted LTH better than FRAIL (P= 0.032). Conclusions Patients diagnosed with frailty had significantly higher 90-day mortality and LTH regardless of which criteria were used. The CFS and FI were better predictors of 90-day mortality, and the CFS was a better predictor of LTH.