Objective The choice of treatment for unprotected left main bifurcation lesions depends largely on the severity of the lesion. SYNTAX score is a kind of severity of coronary artery disease evaluation method, which is based on coronary angiography and a complete anatomical evaluation. In this paper, the total perfusion is regarded as a reference to determine the severity of the lesion. From the perspective of hemodynamics, the severity of different types of bifurcation lesions is assessed. Methods The geometric multi-scale numerical simulation of different bifurcation lesions was carried out by coupling 3D model of the patient and the lumped parameter model. The flow of different branches, wall shear stress( WSS）, oscillatory shear index( OSI） was extracted. By comparing these parameters, the severity of different types of bifurcation lesions was analyzed. Results The results showed that whether the branch (circumflex) the stenosis existed or not had an effect on the total perfusion. When the branch had lesion, the total perfusion volume decreased, which was consistent with the classification of the bifurcation lesion in the SYNTAX score. From the risk of deterioration, when the left main (LM) was stenosis, the average WSS of the bifurcation area was relatively large, but OSI did not show a certain law. Conclusion The total perfusion of branch stenosis is smaller than branch normal. So the degree of myocardial ischemia of branch stenosis is more serious, but the risk of further development of stenosis has no significant difference.