Objective To explore the relationship between individual difference in heparin sensitivity and postoperative bleeding,and lay the foundation for individualized heparin anticoagulant strategy. Methods A total of 120 patients undergoing cardiopulmonary bypass were enrolled in this study. Activated clotting time(ACT)of whole blood was established,and 2.5 mg/kg first heparin was given to the sternum.The value of ACT was measured after five minutes.The patients were divided into 3 groups according to the results of the first measured heparin.If the ACT value did not meet the transfer requirements,heparin 0.5 mg/kg was added,and the amount of bleeding was recorded. The general information of patients,blood transfusion and drainage and other information 24 hours postoperatively were recorded. Results Drainage volume 24 hours after the operation in 3 groups was statistically significant(P < 0.05). Drainage volume 24 hours after the operation in group with ACT>750 s and group with ACT<480 s was significantly higher than that in group with ACT ranging from 480 s to 750 s but the difference be-tween the group with ACT < 480 s and the group with ACT > 750 s was not statistically significant(P > 0.05). Conclusions Heparin sensitivity is associated with postoperative bleeding in patients with cardiopulmonary by-pass surgery,and patients with ACT ranging from 480 s to750 s have the lowest amount of bleeding.