Postoperative delirium is an acute central nervous system dysfunction caused by surgical stress, which is manifested by postoperative acute, non-specific changes in consciousness level, attention, cognitive ability and disturbance of sleep and wake cycles. It is one of the most common surgical complications in the elderly, occurring in more than 75% of patients receiving mechanical ventilation in the intensive care unit(ICU).Exogenous 40 Hz stimulation can improve cognitive functioning.Therefore, the aim of this study was to investigate the effect of exogenous 40Hz stimulation on the incidence of postoperative delirium in elderly patients undergoing elective gastrointestinal surgery in AICU.