Objective:　After　coronary　artery　bypass　grafting　(CABG)　surgery,　the　main　causes　of　poor　instant　patency　of　left　internal　mammary　arteries　(LIMAs)　are　competitive　flow　and　anastomotic　stenosis,　but　how　to　determine　the　cause　of　LIMA　non-patency　without　interfering　with　the　native　coronary　artery　is　still　a　difficult　problem　to　be　solved　urgently.Methods:　In　this　study,　a　0D-3D　coupled　multiscaled　CABG　model　of　anastomotic　stenosis　and　competitive　flow　was　constructed.　After　calculation,　the　flow　waveform　of　the　LIMA　was　extracted,　and　the　waveform　shape,　common　clinical　parameters　(average　flow,　PI,　and　DF),　and　graft　flow　FFT　ratio　results　(F0/H1　and　F0/H2)　were　analyzed.Results:　For　LIMA,　these　three　common　clinical　parameters　did　not　differ　significantly　between　the　anastomotic　stenosis　group　and　competitive　flow　group.　However,　the　waveform　shape　and　FFT　ratio　(especially　F0/H2)　of　the　competitive　flow　group　were　significantly　different　from　those　of　the　anastomotic　stenosis　group.　When　the　cause　was　competitive　flow,　there　was　systolic　backflow,　and　F0/H2　was　too　high　(＞14.89).　When　the　cause　was　anastomotic　stenosis,　the　waveform　maintained　a　bimodal　state　and　F0/H2　was　in　a　normal　state　(about　1.17).Conclusion:　When　poor　instant　patency　of　the　LIMA　is　found　after　CABG,　the　causes　can　be　determined　by　graft　flow　waveform　shape　and　F0/H2.