Abstract: Objective To observe the influence of dexmedetomidine (DEX) on postoperative cognitive function and BDNF/TrkB signaling pathway in elderly patients undergoing total hip replacement . Methods Sixty patients over age 60 were randomly divided into three groups, DEX low dose group (A), DEX high dose group (B) and control group (C). Patients in A and B were loaded 0.5 μg DEX in 10 min, and C were loaded saline prior to anaesthesia. After induction of anesthesia, patients in A and B were injected 0.25μg/(kg?h) and 0.5μg/(kg?h) DEX, respectively, C were injected saline , all patients injected an hour.The vein blood were collected before DEX injection (T1), one hour injected (T2), and one day (T3) after operation to detect BDNF/ TrkB. The blood samples of the radial artery and the internal jugular vein were collected, and the blood gas analysis was performed to calculate the Da - jv02 and CERO2. The MMSE score were recorded at one day before operation and seven days after operation. Results The BDNF and TrkB expression in A group and B group was higher than C group at T2 and T3 (P < 0.05). moreover, the BDNF and TrkB expression in B group was higher than A group at T2 and T3 (P < 0.05). The Da-jv02 and CERO2 were lower at T2 in A group and B group compared to C group (P < 0.05) , and the MMSE in A group and B group was increased compared to C group. Conclusion The DEX could reduce the rate of POCD in elderly patients undergoing total hip replacement, and the reason might related to reduction of Da-jv02 and CERO2 and increasing expression of BDNF and TrkB