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Introduction Postoperative delirium (POD) is a common acute central nervous system syndrome in elderly patients following surgery. Due to pre-existing brain tissue damage, elderly patients with non-acute fragile brain function (NFBF) are at higher risk of developing POD, which can lead to long-term cognitive impairment, postoperative complications increasing and prolong hospitalisation. Transcutaneous auricular vagus nerve stimulation (taVNS) has been demonstrated to play a significant role in treating various neurological disorders, including epilepsy, depression and consciousness impairment. However, its efficacy in preventing POD in elderly NFBF patients remains unknown. Therefore, this study aims to investigate the effect of perioperative taVNS on the incidence of POD in elderly NFBF patients.Methods and analysis A bi-centre, double-blind, randomised, controlled trial will be conducted at Beijing Friendship Hospital, Capital Medical University and Xiyuan Hospital of China Academy of Traditional Chinese Medical Sciences, China, from December 2025 to December 2026. A total of 148 elderly NFBF patients (≥60 years old) undergoing abdominal surgery will be enrolled and randomly allocated to two groups: the taVNS group (receiving transcutaneous auricular vagus nerve stimulation) and the tnVNS group (receiving transcutaneous non-vagus nerve stimulation). The primary outcome will be the incidence of POD within 5 days after surgery. Secondary outcomes include delirium severity, changes in biomarkers, postoperative delayed cognitive dysfunction, Numerical Rating Scale (NRS) scores, recovery of gastrointestinal function, anaesthesia-associated parameters, economic indicators, and the incidence of perioperative complications or other serious adverse events.Ethics and dissemination This study was approved by the Ethics Committee of Beijing Friendship Hospital, Capital Medical University and adheres to the principles of the Declaration of Helsinki. The protocol was written in accordance with the 2013 Standard Protocol Items: Recommendations for Interventional Trials guidelines. The results of this study will be published in a peer-reviewed journal and presented at national or international conferences.Trial registration number ITMCTR2025002263.