Abstract: To numerically investigate and compare blood flow in arteries under different boundary conditions, the selection of boundary conditions in hemodynamics was analyzed,and then the certain solution conditions were presented.On the basis,the parameters of thoracic aorta were employed,and numerical calculations of steady and unsteady flows were performed under different inlet and outlet conditions respectively.The results show that the pressure distributions are uniform and the parabolic velocity profile is gradually changed to be flat for steady flow,the pressure profile is gradually converted from the inlet pressure profile to the outlet pressure profile and the velocity distributions are blunt for unsteady flow.Boundary conditions have a significant effect on flow field distributions.