A polarization converter that uses metasurfaces composed of cross deformable structure is proposed. Ultrawideband 90° polarization rotation is obtained in the reflective mode. Reflectance of the cross-polarized wave is above -0.2 dB while co-polarization wave is blow -12 dB in range of 8.4-20.7 GHz. The cross-polarized reflectance is greater than -0.03 dB at the resonant frequency points, and the co-polarized reflectance reaches -60 dB. That is, almost complete cross-polarization conversion be achieved at the resonance frequency points. The relative bandwidth is 84.5% with average orthogonal polarization conversion ratio (PCR) of 96.7%. In addition, the metasurface polarization converter is fabricated on FT5880 substrate. The experimental results show that the cross-polarized reflectance is greater than -1 dB in the operating frequency band and the co-polarized reflectivity is less than -10 dB. The experimental results are in good agreement with the simulation results, which verifies that the metasurface can realize the cross-polarization conversion of the linearly polarized electromagnetic wave in the ultra-wide frequency band. The ultrawideband conversion metasurface has the advantages of high conversion efficiency and simple geometric structure. The similar model can be extended to terahertz or even visible light.