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Objective To analyze the influence of competitive flow on hemodynamics of left internal mammary artery graft after coronary artery bypass grafting (CABG) with different afterload. Methods Three-dimensional (3D) models of left internal mammary artery (LIMA) to left anterior descending (LAD) grafting the were reconstructed, and 1 afterload and 1.2 afterload boundary conditions of these models were provided by the lumped parameter model (LPM, 0D) of cardiovascular circulatory system. Meanwhile, five different stenoses at the trunk of LAD were reconstructed, including 25%, 40%, 50%, 60%, 75%. Finally, 3D model and 0D model were coupled by computational fluid dynamics (CFD) to observe the influence of competitive flow with different afterload on LIMA. Results For 1 afterload, flow rate of LAD was 80mL/min, and with stenosis increasing, RC/G decreased from 2.025 to 0.280, mean flow rate of LIMA ranged from 26.598mL/min to 62.310mL/min, oscillatory shear index (OSI) decreased from 0.1557 to 0.0020, and time-average wall shear stress (TAWSS) was <1Pa when the stenosis was less than 50%. For 1.2 afterload, flow rate of LAD was 71mL/min, and with stenosis increasing, RC/G decreased from 2.222 to 0.289, mean flow rate of LIMA ranged from 22.188mL/min to 54.810mL/min, OSI decreased from 0.1790 to 0.0024, and TAWSS was <1Pa when the stenosis was less than 60%. When the afterload of coronary was different, flow rate in systole changed little with the same stenosis, however in diastole, flow rate of LIMA decreased with the afterload increasing. Conclusions When the afterload of coronary increases, intensity of competitive flow increases. If the vascular diameter and stenosis of coronary are all the same for different patients, “string phenomenon” may appear in LIMA graft with high afterload because of lower WSS and higher OSI, which may reduce the long-term patency of the graft.