In　order　to　research　blood　flow　in　Y-bifurcation　influence　on　thrombosis,　a　blood　flow　model　in　the　Y-Bifurcation　coronary　arteries　is　set　up　based　on　Navier-Stokes　flow　of　the　viscous　steady　fluid.　Blood　flow　in　the　difficult　Y-Bifurcation　coronary　arteries　is　numerically　simulated　and　influences　of　angle　of　bifurcation,　blood　viscidity　and　initial　blood　velocity　of　inlet　on　velocity　field　of　blood　flow,　shear　stress　and　pressure　on　the　wall　as　well　as　the　formation　of　thrombosis　are　also　investigated　by　making　use　of　computational　fluid　dynamics　(CFD)　method.　It　is　shown　from　the　results　that　locations　apt　to　thrombus　or　the　best　locations　to　pathological　change　are　in　the　crotch　of　Y-Bifurcation.　Some　guesses　of　thrombosis　formation　in　medicine　research　are　also　proved　by　the　results　in　the　paper.