A reflective polarization converter constructed by a layer of rectangle metal pattern, an ultrathin polyimide substrate, and a layer of metal reflector are introduced. To lower the sensitivity to the thickness of the substrate and increase the working bandwidth, the method of broadening and truncating the metallic wire grid of traditional polarization converter is proposed, which introduces the equivalent capacity and the equivalent inductance of two orthogonal linearly polarized incident electromagnetic waves, so the phases are modulated separately. The simulation result suggests that at 500 GHz, the working bandwidth is about 140 GHz, the insert loss is less than 0.5 dB; and while the thickness error of the polyimide substrate is within a due range, the performance of the polarization converter would keep stable. The designed converter efficiently lowers the sensitivity to the thickness of the substrate and increases the working bandwidth, besides, a flexible thin-film photoetching method is proposed for the process of the polarization converter.