Objective To study the flow field of a new vascular bioreactor with small diameter at different operating conditions. Method To investigate the flow field of a new vascular bioreactor with small diameter under three operating conditions by numerical simulation: outerbox rotating, innerbox rotating and both innerbox and outerbox rotating. A comparison of parameters such as flow velocity and shear stress was made. Results The culture medium in the bioreactor rotated with the innerbox and outerbox, and the velocity was uniformly distributed; the bioreactor could therefore provide a low shear stress environment that is nondestructive for the cells in the medium; the shear stress in bioreactor had a quadratic function relation with the rotation rate. Conclusions This bioreactor could provide a good culture environment for vascular tissue regeneration, and this study provided theoretical basis for the experimental research of bioreactors.