OBJECTIVE： To investigate the effects of dexmedetomidine on myocardial injury and postoperative cognitive function in patients with cardiopulmonary bypass （CPB） valve replacement. METHODS： A total of 90 patients underwent elective CPB valve replacement in our hospital during Jan. 2015-Dec. 2017 were divided into group C and group D according to random number table， with 45 cases in each group. Group D was given Dexmedetomidine hydrochloride injection 0.6 μg/kg after anesthesia induction and intravenous injection at 0.6 μg/（kg·h） to the end of the operation； group C was given 0.9％ Sodium chloride injection at equal volume and rate. SBP， DBP， HR and MAP were observed in 2 groups immediately before medication （T0）， immediately after medication （T1）， immediately after incision （T2）， immediately after sternotomy （T3）， immediately before CPB （T4）， immediately after CPB stopping （T5） and immediately after operation （T6）， respectively. The levels of CK-MB， H-FABP and cTnⅠ were observed at T0， T5， T6， at 6 h （T7） and 24 h （T8） after operation. The levels of S-100β protein and NSE were observed at T0， T8 and 72 h after operation （T9）. MMSE and MoCA scores were observed 1 d before operation and 3， 7 d after operation. The application of cardiovascular active drugs and the occurrence of ADR were observed during operation. RESULTS： SBP and DBP of group C at T2-T6， SBP at T4-T6 and DBP at T4 of group D were significantly lower than at T0； SBP and DBP of group D were significantly higher than group C at T4-T6. HR of 2 groups at T4 were significantly lower than at T0， while those of 2 groups at T5-T6 were significantly higher than at T0； the group D were significantly lower than group C at T2-T3. MAP of 2 groups at T2-T3 were significantly higher than at T0， and those of group D were significantly higher than group C at T4， with statistical significance （P＜0.05）. At T5-T8， the levels of CK-MB， H-FABP and cTnⅠ in 2 groups were significantly higher than at T0； the levels of CK-MB and cTnⅠ at T7-T8， the level of H-FABP at T5-T8 in group D were significantly lower than group C， with statistical significance （P＜0.05）. At T8-T9， the levels of S-100β protein and NSE in 2 groups were significantly higher than at T0， but the group D was significantly lower than group C， with statistical significance （P＜0.05）. MMSE scores and MoCA scores of 2 groups 3 d after operation， MMSE score and MoCA score of group C 7 d after operation were significantly lower than 1 d before surgery； those of group D 3， 7 d after operation were significantly higher than group C， with statistical significance （P＜0.05）. The amount of dopamine and norepinephrine， the rate of adrenalin use in group D were significantly lower than group C， with statistical significance （P＜0.05）. There was no statistical significance in the amount of milrinone between 2 groups （P＞0.05）. The incidence of ADR in group D （6.7％） was significantly lower than group C（24.4％）， with statistical significance（P＜0.05）. CONCLUSIONS： Dexmedetomidine is helpful for hemodynamic stability， relieve myocardial damage， and improve postoperative cognitive dysfunction in patients with CPB valve replacement with good safety.