Background:　During　coronary　artery　bypass　grafting　(CABG),　the　ratio　of　powers　of　the　fundamental　frequency　and　its　first　harmonic　(F0/H1)　in　fast　Fourier　transformation　(FFT)　analysis　of　the　graft＇s　flow　waves　has　been　used　in　the　field　of　evaluation　of　the　patency　in　anastomosis.　But　there　is　no　report　about　using　the　FFT　method　to　evaluate　the　magnitude　of　competitive　flow.　This　study　is　aiming　at　exploring　the　relationship　between　competitive　flow　and　FFT　analysis　of　the　flow　waves　in　left　internal　mammary　artery　(LIMA)　graft,　and　finding　a　new　method　to　evaluate　the　magnitude　of　competitive　flow.　Methods:　At　first,　establishing　the　CABG　multiscale　models　of　different　stenosis　in　left　anterior　descending　artery　(LAD)　to　get　different　magnitude　of　competitive　flows.　Then,　calculating　the　models　by　ANSYS-CFX　and　getting　the　flow　waves　in　LIMA.　Finally,　analyzing　the　flow　waves　by　FFT　method　and　comparing　the　FFT　results　with　the　magnitude　of　competitive　flow.　Results:　There　is　no　relationship　between　competitive　flow　and　F0/H1.　As　for　F0/　H2　and　F0/H3,　they　both　increase　with　the　reduction　of　the　stenosis　in　LAD.　But　the　increase　of　F0/H3　is　not　obviously　enough　and　it　can＇t　identify　the　significant　competitive　flow　clearly,　so　it　can＇t　be　used　as　the　evaluation　index.　It　is　found　that　F0/　H2　increases　obviously　with　the　increase　of　the　competitive　flow　and　can　identify　the　significant　competitive　flow.　Conclusion:　The　FFT　method　can　be　used　in　the　evaluation　of　competitive　flow　and　the　F0/H2　is　the　ideal　index.　High　F0/H2　refers　to　the　significant　competitive　flow.　This　method　can　be　used　during　CABG　to　avoid　the　risk　of　competitive　flow.