Objective:To evaluate the effect of Danhong Injection(丹红注射液,DH)on the index of microcirculatory resistance(IMR)and myocardial injury in patients with unstable angina undergoing elective percutaneous coronary intervention(PCI).Methods:Seventy-eight patients with unstable angina were randomly divided into DH group(39 cases)and the control group(39 cases)during elective PCI.Randomization was performed using a random-number table.The DH group received DH at a dosage of 40 mL(mixed with 250 mL saline,covered by a light-proof bag,intravenous drip)during PCI and daily for 7 consecutive days,while the control group only received the same dosage of saline.Both groups received standardized treatment.The IMR and fractional flow reserve(FFR)were measured at maximal hyperemia before and after PCI.Myocardial markers,including myoglobin,creatine kinase(CK),creatine kinase MB(CK-MB),and coronary troponin T(cTnT)values were measured at baseline and 24 h after PCI.Results:Among the 78 patients enrolled,the baseline and procedural characteristics were similar between the two groups.There was no significant difference in pre-PCI myocardial markers and coronary physiological indexes between the two groups.However,post-PCI CK and CK-MB levels in the DH group were significantly lower than those in the control group(111.97±80.97 vs.165.47±102.99,P=0.013;13.08±6.90 vs.19.75±15.49,P=0.016).Post-PCI myoglobin and cTNT-positive tend to be lower in the DH group than in the control group but did not reach statistical significance(88.07±52.36 vs.108.13±90.94,P=0.52;2.56％vs.7.69％,P=0.065).Compared with the control group,the post-IMR levels of the DH group tended to decrease,but there was no statistical difference(20.73±13.15 vs.26.37±12.31,P=0.05).There were no statistical differences in post-FFR in both groups.The peri-procedural myocardial injury of the DH group was significantly lower than that of the control group(2.56％vs.15.38％,P=0.025).During the 30-d follow-up period,no major adverse cardiovascular events occurred in either group.Conclusion:This study demonstrated benefit of DH in reducing myocardial injury and potential preserving microvascular function in patients with unstable angina undergoing elective PCI.