Warpage is one of the main defects in multi-pass roll forming. It manifests as the horizontal deviation in the sheet flange part after forming, impacting the final quality of the sheet. In this manuscript, we focus on the cap-shaped part of a small section profile. Considering the lack of previous scientific guidance on the distribution of forming angles—angles that lead to warpage defects and other defects reducing the quality of these products—a five-boundary condition distribution function of forming angle is proposed. We propose a mechanism for warpage defects in roll bending of a cap-shaped part based on this five-boundary condition forming angle distribution function. Furthermore, we examine the effects of forming angle, sheet thickness, and material yield strength on the warpage defects by examining the designed fluctuation of the edge wave, ∆z, and the maximum deviation of the profile curves of the flange edge, z', to test this theory, roll bending experiments are compared to simulation.