Objective To investigate the influence of graft-host diameter ratio on the flow field of fully-occluded artery bypass grafts and provide a theoretical guidance for reducing restenosis in artery bypass graft surgery. Methods Five models were employed to numerically investigate the influence of graft-host diameter ratio on the flow field of fully-occluded artery bypass grafts. The distributions of hemodynamic parameters such as velocity, second flow, wall shear stress (WSS) and wall shear stress gradient (WSSG) and their change in line with the increase of diameter ratio were analyzed. In addition, comparison of hemodynamic differences in a fully-occluded complete model (Model A), a fully-occluded partial model (Model B) and a 75% stenosis complete model (Model C) with graft-host diameter ratio of 1.0 was conducted to validate the fully-occluded complete model established in this study. Results The hemodynamic performance of Model A was totally different from that of Model C, and the velocity distribution at the graft top had an obvious influence on the WSS distribution at host artery bed in the downstream anastomosis, with maximum WSS differences reaching 79%. A large graft-host diameter ratio resulted in a large size of low WSS region at the host artery bed, but with uniformly distributed WSS and small WSSG. A small graft-host diameter ratio resulted in a small size of low WSS region at the host artery bed, but with large WSSG. Conclusions It is necessary to adopt a complete model to study the influence of graft-host diameter ratio on the flow field of fully-occluded artery bypass grafts. The diameter ratio had a significant impact on the flow field of fully-occluded artery bypass graft, thus a large ratio could be helpful to reduce the occlusion resulted from the restenosis at the downstream anastomosis in artery bypass graft.