<p>Flexible skew rolling (FSR) of hollow shafts with a mandrel represents a novel near-net-forming technology for hollow shafts. Surface quality, particularly the presence of spiral mark defects, poses a significant challenge in achieving precision forming. In this paper, the formation mechanism of spiral marks of hollow FSR shaft with mandrel was studied through experimental methods and finite element (FE) simulations, and the morphology of spiral marks under different rolling parameters is analyzed. Our findings indicate that the initiation of spiral marks occurs at the point where the rolled piece separates from the rolls. The outer spiral marks are attributed to the mismatch between the radial and axial metal flow; when the rolled part separates from the rolls, the metal that has exited the rolls is influenced by the deforming metal still within the rolls, resulting in an accumulation of excess material that takes on a spiral shape, mirroring the profile of the rolled piece. The intensity of spiral marks increases with higher swing angles, greater reduction ratios, and larger mandrel diameters, while decreasing with an increase in relative wall thickness. The spiral mark defect could be mitigated by extending the sizing section length, incorporating the unloading fillet and selecting appropriate rolling parameters. When the roll sizing length increased from 20 to 30&#xa0;mm and the unloading fillet is set at 5&#xa0;mm, the depth of spiral marks was improved by 21.8%. The results elucidate the causes of spiral marks on hollow shafts produced by FSR with a mandrel and provide theoretical guidance for selecting process parameters in production applications.</p>