Background　and　Aims:　Competitive　flow　exists　in　the　native　coronary　artery　when　using　artery　graft　in　the　coronary　artery　bypass　graft　(CABG),　and　it　is　a　major　factor　affecting　the　long-term　patency　of　the　arterial　grafts.　The　purpose　of　this　study　is　to　investigate　the　effects　of　the　competitive　flow　on　hemodynamic　characteristics　of　left　internal　mammary　artery　(LIMA).　Materials　and　methods:　Seven　three-dimensional　(3D)　LIMA-left　anterior　decending　(LAD)　grafting　models　with　different　diameter　stenoses　at　the　trunk　of　LAD　were　reconstructed,　including　25%,　40%,　50%,　60%,　75%,　90%　and　100%.　The　boundary　conditions　of　these　models　were　provided　by　the　same　lumped　parameter　model　(LPM;　0D　sub-model)　of　cardiovascular　circulatory　system,　which　is　based　on　0D/　3D　coupling　method.　Results:　The　simulation　results　show　that　competitive　flow　decreases　when　the　stenosis　rate　increases　and　backflow　exists　in　LIMA　during　systole　when　the　degree　of　stenosis　is　below　50%;　the　wall　shear　stress　(WSS)　of　the　graft　increases　from　0.85　Pa　to　1.79　Pa　with　the　increase　of　the　native　coronary　stenosis　degree,　while　the　oscillatory　shear　index　(OSI)　decreases　from　0.1557　to　0.00213.　Conclusions:　Competitive　flow　may　produce　adverse　hemodynamic　effects　on　artery　graft,　especially　when　the　stenosis　degree　is　below　50%,　competitive　flow　will　significantly　lead　the　long-term　patency　of　the　artery　graft　worse　because　of　the　backflow,　low　WSS　and　high　OSI,　and　the　effect　of　OSI　is　more　obvious　than　WSS.