OBJECTIVE： To investigate the effects of early before PCI and immediate application of tirofiban on coronary blood flow and myocardial perfusion in patients with high risk acute coronary syndrome （ACS）. METHODS： 100 high risk ACS patients were randomly divided into observation group and control group， with 50 cases in each group. Two groups were given tirofiban； observation group was given medicine 10 μg/kg within 3 min， 4-6 h before PCI， with the velocity of 0.15 μg/（kg·min） till 24 h after PCI. Control group was given medicine at the beginning of PCI， route of administration was same as observation group. The incidence of TIMI blood flow grading， TIMI myocardial perfusion grade （TMPG）， cTn Ⅰ， PAR and main adverse cardiac events （MACE） were compared between 2 groups before and after treatment. RESULTS： After operation， the patients of 2-3 grade TIMI blood flow and 2-3 grade TMPG in 2 groups were significantly more than before； the patients of 2-3 grade TMPG in observation group was significantly more than in control group， with statistical significance （P＜0.05）. After treatment， cTn Ⅰof 2 groups were significantly increased， while PAR were significantly decreased， with statistical significance compared to before operation （P＜0.05）； but there was no difference between 2 groups （P＞0.05）. There was no statistical significance in the incidence of MACE between observation group （8.0％） and control group （16.0％） （P＞0.05）. CONCLUSIONS： Early application of tirofiban before PCI can effectively improve coronary blood flow and myocardial blood supply in high risk ACS patients.