Objective To explore the correlation between preoperative frailty and severe postoperative complications within postoperative 30 d after abdominal surgery in elderly patients and the underlying mitochondrial mechanisms. Methods A total of 256 patients who were scheduled for abdominal surgery at West China Hospital of Sichuan University from September 2019 to October 2020 were selected. A modified frailty index (MFI) scale was used to evaluate preoperative frailty. Meanwhile, their baseline information, including age‐adjusted Charlson comorbidity index (aCCI), was calculated and peripheral blood mononuclear cells were collected. The bioenergetic health index (BHI) was measured by a Seahorse XF24 analyzer for the mitochondrial stress test. They were followed up on serious postoperative complications until 30 d after surgery. Primary outcome was the correlation between preoperative frailty and serious complications within postoperative 30 d. Secondary outcomes were: ① Difference of the following postoperative overcome information between frailty and non‑frailty patients, including the incidence of severe complications within postoperative 30 d, comprehensive complication index (CCI) scores, mortality with postoperative 30 d, the length of postoperative hospitalization/intensive care unit (ICU) stay, ICU admission and total medical expense. ② The correlation between BHI and frailty and serious complications within postoperative 30 d. Results Frailty patients showed higher aCCI scores than non‑frailty patients before surgery (P<0.05), and there was no statistical difference in other baseline data between the two groups (P>0.05). According to univariate Logistic regression, frailty was associated with severe complications within postoperative 30 d ［odds ratio (OR) 5.89 (95%CI 2.64, 13.14), P<0.05］. The multivariate regression after adjusting for variables also showed that frailty was associated with severe complications within postoperative 30 d ［OR 5.86 (95%CI 2.34, 14.68), P<0.05］. The frailty patients had an increased incidence of severe complications within postoperative 30 d and CCI scores, extended length of postoperative hospitalization stay, and increased overall medical expense, compared with non‑frailty patients (P<0.05). However, no statistical difference was found in mortality within postoperative 30 d, ICU admission, and length of ICU stay between the two groups (P>0.05). There was no significant difference in BHI between frailty and non‑frailty patients (P>0.05). Also, BHI was not associated with frailty or serious complications within postoperative 30 d. Conclusions Preoperative frailty in elderly patients is related to serious complications within postoperative 30 d. The surgical outcome of frailty patients is worse than non‑frailty patients. In addition, BHI, which reflects mitochondrial function, is not suitable for predicting serious complications within postoperative 30 d.