Abstract: 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.