Objective To investigate the influence of competitive flow at different lelves on wall shear stress (WSS) of left internal mammary artery graft after coronary artery bypass grafting. Methods The left internal mammary artery (LIMA)left anterior descending (LAD) anastomotic model was extracted and reconstructed from CT images based on thresholding method by using SimVascular software. The competitive flow was divided into three groups according to different stenosis of LAD, including no competitive flow group (100% stenosis of LAD), mild competitive flow group (50% and 75% stenosis of LAD) and severe competitive flow group (0% and 30% stenosis of LAD). The hemodynamic performace of the anastomotic model under different conditions of competitive flow was observed by computational fluid dynamics (CFD) method. Results With the increment of competitive flow, the value of WSS was decreased gradually while the oscillation of WSS was increased remarkably in LIMA graft. The time average WSS (TAWSS) of LIMA graft in severe competitive flow group (0% stenosis: 2.73 Pa, 30% stenosis: 2.85 Pa) was lower than that in the mild competitive flow group (50% stenosis: 4.77 Pa, 75% stenosis: 6.01 Pa) and no competitive flow group (100% stenosis: 8.64 Pa), while its oscillatory shear index (OSI) (0% stenosis: 0.206; 30% stenosis: 0.085) was much higher than that in other two groups (50% stenosis: 0.014; 75% stenosis: 0.013; 100% stenosis: 0.006). Conclusions When the stonosis of LAD was smaller than 50%, the WSS in LIMA graft was obviously lower and oscillatory due to severe competitive flow. Such unfavorable feature of WSS may influence the long term-patency of LIMA graft and long term-survival of operations.