Objective To evaluate the effect of frailty on postoperative complications in elderly patients with esophageal cancer.Methods We enrolled the patients aged≥65 years and with esophageal cancer who underwent surgical treatment in Shanghai Chest Hospital in 2021. The modified frailty index (mFI) was calculated and the patients were divided into a non-frailty group and a frailty group. The primary outcomes were the incidence of postoperative pulmonary infection, arrhythmia, anastomotic fistula and chylothorax complications. Secondary outcomes were the time of extubation, the rate of unplanned re-intubation, the length of ICU stay, hospital stay, rate of readmission within 30 days after discharge and the mortality within 30 days after operation.Results Finally 607 patients were collected. There were 273 patients in the non-frailty group and 334 patients in the frailty group. The non-frailty group had lower rates of complications including pulmonary infection (5.5% vs. 13.5%), arrhythmia (3.7% vs. 9.3%), anastomotic fistula (2.9% vs. 7.5%), and shorter ICU stay [2.0 (0.0, 4.0) d vs. 4.0 (1.0, 6.0) d] and in-hospital stay [11.5 (9.5, 13.0) d vs. 13.0 (11.0, 18.0) d],lower rates of the readmission within 30 days (2.9% vs. 6.6%) and the mortality within 30 days (0.4% vs. 1.2%) compared with the frailty group (P<0.05).Conclusion Frail elderly patients with esophageal cancer have higher rates of postoperative complications. mFI can be used as an objective index to identify high-risk elderly patients with esophageal cancer.