Objective To observe the effects of different administration doses of dexmedetomidine on the circulatory system and stress response in patients undergoing extracorporeal coronary artery bypass grafting （OPCABG）. Methods Ninety-six patients who underwent OPCABG in our hospital from October 2021 to October 2023 were selected and divided into two groups using simple randomization method. Group A was administered dexmedetomidine at a dose of 0.5 μg/kg over 10 minutes before anesthesia induction， followed by a maintenance dose of 0.5 μg/（kg·h） infused until the end of the surgery， while group B was administered dexmedetomidine at a dose of 0.8 μg/kg for 10 min before anesthesia induction， followed by a maintenance dose of 0.8 μg/（kg·h） until the end of the operation. The two groups were compared in terms of cardiac index （CI）， heart rate， mean arterial pressure （MAP）， intrathoracic blood volume index （ITBI）， bispectral index （BIS）， and systemic peripheral vascular resistance index （SVRI） before anesthesia started （T0）， at the immediate moment of intubation （T1）， at the immediate moment of the start of the surgery （T2）， at the time of sawing of the sternum （T3）， and at the immediate moment of extubation （T4）. Additionally， the two groups were compared in terms of cortisol （Cor）， Angiotensin Ⅱ （Ang Ⅱ） levels， safety and postoperative awakening time. Results The two groups showed no differences in operation time， anaesthesia time， bleeding and urine volume （P > 0.05）， but group B demonstrated less intraoperative use of cisatracurium compared to group A （P < 0.05）. At T0， the two groups showed no difference in heart rate and MAP （P > 0.05）， but higher heart rate and MAP at T1 and T3 than at T0. Group A was observed to have higher heart rate and MAP at T4 than at T0， while group B showed no significant differences in heart rate and MAP at T4 compared to them at T0 （P > 0.05）. At T2 and T4， Group B showed significantly lower heart rate and MAP compared to group A （P < 0.05）. At T0， the two group had no differences in terms of CI， CO， ITBI， and SVRI （P > 0.05）. Both groups showed significantly lowered levels of CI， CO， ITBI， and SVRI at T1-T4 than at T0 （P < 0.05）， but demonstrated no differences in the levels of CI， CO， ITBI， SVRI at T0-T4 （P > 0.05）. At T0， both groups had no difference in BIS values （P > 0.05）， but showed significantly decreased BIS values at T1-T4 compared with those at T0 （P < 0.05）. At T2-T4， group A showed significantly lower BIS values compared with group A （P < 0.05）. The two groups had no difference in postoperative awakening time compared with group A （8.12 ± 1.88 min vs. 8.05 ± 1.97 min， P > 0.05）. Preoperatively， the two groups had no differences in Cor and Ang Ⅱ（P > 0.05）. However， at 6 h postoperatively， both groups showed significantly elevated Cor and AngⅡ values compared to preoperatively （P < 0.05）， and group B showed signifantly lower values of Cor and AngⅡ compared to group A （P < 0.05）. The two groups had no difference in the adverse reactions （6.25% vs. 8.33%， P > 0.05）. Conclusion Dexmedetomidine administered at the dose of 0.8 μg/（k·h） rather than at the dose of 0.5 μg/（k·h） for managing OPCABG results in more stable hemodynamics during surgery， yielding better sedative effect， milder postoperative stress response， and no increase in adverse reactions.