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.