Objective To investigate the mechanical mechanism of bypass graft for the treatment of DeBakey III aortic dissection and explore the valid surgical planning. Methods Patient-specific models of DeBakey III aortic dissection, including the models of through lumen and blind lumen, before and after bypassing between ascending aorta and abdominal aorta, between left subclavian artery and abdominal aorta, were constructed, and then numerical simulations were performed using computational fluid dynamics (CFD) method under physiological flow conditions based on fluid-structure interaction (FSI). Results Blood flow velocity, pressure, vessel wall displacement of the false lumen after bypass graft were reduced by 38.86%, 15.347 kPa and 39.46% on average, respectively. Conclusions Bypass graft is an effective surgical method for the treatment of DeBakey III aortic dissection under specific conditions with good prospects in clinical application.