In this paper, by using a practical 3D FE model, one power spinning process of parts with transverse inner rib has been simulated. The stress and strain fields have been obtained and the features of metal flow in plastic deformation zone have been discussed thoroughly. Then, the main forming defects and their origins have been explored based on the study of the metal flow, shown as follows; (1) during the non-rib process, the main forming defect is the wall fracture in finished deformation zone. The origin of this forming defect is the excessive metal flow in thickness direction ahead of the forming roller. (2) in the rib-forming process, the main forming defect is the undesired inner rib. The origin of this forming defect is the excessive metal flow in hoop direction during the former non-rib process. (3) the features of the metal flow during flange part process are almost the same as those in the non-rib process. The achievements of this study can thoroughly reveal the deformation mechanism and provide an important basis for the optimization of the process parameters and the precise control of the power spinning process of parts with transverse inner rib.