Transit time flow-meter (TTFM) has been usually used in coronary artery bypass grafting (CABG) to detect whether a graft is patent or not, but it can't determine the reason of the graft's non-patency. The graft's non-patency is mainly caused by two reasons: competitive flow and anastomotic stenosis. To solve this problem, several CABG multi-scale models with different degrees of competitive flows or anastomotic stenosis have been constructed in this study to calculate their graft flow waves. The shape of the waveform, the average flow, pulsating index (PI), diastolic velocity time integral fraction (DF) and fast Fourier transformation (FFT) ratio have been obtained from calculation results. According to the analysis, the shape of the waveform and FFT ratio could be the indexes to determine whether the reason of the non-patency is competitive flow or anastomotic stenosis.