OBJECTIVE： To investigate the effects of dexmedetomidine on postoperative cognitive dysfunction （POCD） and perioperative T lymphocyte subsets in senile patients after total hip joint replacement. METHODS： 172 senile patients receiving hip joint replacement were randomly divided into observation group and control group， with 86 cases in each group. Both groups was given combined spinal anesthesia， and observation group was additionally given dexmedetomidine. Postoperative cognitive functional status and perioperative T lymphocyte subpopulation changes were observed in 2 groups. RESULTS： There was no significant difference of MMSE score 1 day before surgery， 3 and 7 days after surgery in observation group （P＜0.05）. MMSE score in control group significantly decreased 3 days after surgery， there was statistical significance compared to before surgery （P＜0.05） and recovered to normal level 7 days after surgery. The incidence of POCD in observation group was 9.3％， which was significantly lower than （22.1％） in control group， with statistical significance （P＜0.01）. In the two groups， serum IL-1β， IL-6 and TNF-α levels were significantly higher than that before anesthesia 5 min after induction， once after the withdrawal of maintaining drug； this was more significant in control group， with statistical significance （P＜0.05）. Serum inflammatory factors in observation group recovered to preoperative level 1 h after surgery， while serum inflammatory factors in control group were still significantly higher than that before anesthesia， with statistical significance （P＜0.05）. 5 min after anesthesia induction， CD3， CD4， CD4/CD8 were significantly higher than that before anesthesia； this was more significant in control group， with statistical significance （P＜0.05）. CONCLUSIONS： Dexmedetomidine plays a role of auxiliary sedation in senile patients during combined spinal anesthesia total hip joint replacement， has a certain effect on reducing POCD incidence. This is related to its effect of reducing inflammation and maintaining the stability of T lymphocytes.