【Abstract】 Objective To study effect and mechanism of dexmedetomidine on postoperative delirium in elderly patients undergoing thoracoscopic surgery. Methods 120 elderly patients undergoing thoracoscopic surgery in our hospital from January 2015 to January 2017 were selected. The patients were divided into the control group and the observation group randomly, according to the digital random method. The two groups of patients were given the monitoring of oxygen saturation under general anesthesia. And the observation group was given dexmedetomidine for injection of 15min at 05 micron g/kg before the induction of anesthesia, followed by continuous infusion of 03 ~ 05 μg/kg/h and 30min stopped infusion until the end of operation. The control group was given the same dose of physiological saline.Before induction(T0), before intubation (T1), one lung ventilation (T2), before the end of one lung ventilation (T3), before extubation (T4), 5min after extubation (T5) mean arterial pressure (mean arterial pressure, MAP), cerebral oxygen saturation (Cerebral regional oxygen saturation, rSO2) were compared.Record the time of operation and the dosage of narcotic drugs.At the end of the operation, and after the operation of 1D and 3D serum S100 beta protein levels were detected at the end of the anesthesia according to the DSMIV delirium / delirium evaluation criteria for the diagnosis of postoperative delirium. Then the serum s100β protein level and cerebral regional oxygen saturation (rSO2) of the two groups were compared. Results Fentanyl, propofol, atracurium perphenol sulfonic acid, sevoflurane, and the difference of operation time were no statistically significant in two groups（P＞005）.The usage remifentanil was lower in observation group than that in the control group（P＜005）. T3rSO2 was significantly higher in observation group than that in the control group,and T2 MAP was lower in observation group than that in the control group（P＜005）. Incidence of delirium was 466% after operation in observation group,and lower than this in control group（1500%）（P＜005）. The serum s100β levels in the observation group at the end of surgery and 1 d and 3 d after operation were significantly lower than those in the control group (P＜005). The mean rSO2 in the observation group was significantly higher than that in the control group (P＜005). Conclusion Dexmedetomidine on Monitoring of cerebral oxygen saturation can significantly decrease the incidence of postoperative delirium,reduce the level of serum S100 beta and improve the metabolism of brain oxygen in the operation.