The polarization state is one of the most important basic properties of the electromagnetic wave. Researchers have made great efforts to manipulate it. Control of the polarization state of an electromagnetic wave is a promising promotion for figuring out many practical engineering problems in infrared remote sensing, optical communication and infrared target recognition. In this paper, we propose a wide-band and high-efficient linear-polarization converter on the basis of the metamaterial, which is comprised of silicon nanorod array and subwavelength metal grating that can realize a 90 polarization converter of linearly polarized light and is composed of silicon nanorod array cascade subwavelength metal grating:on one side of design located is the cuboid silicon nanorod array, on the other side of the design the subwavelength metallic grating on the silicon substrate, and the angle between silicon nanorod array and subwavelength metal grating is 45. Because of the deference in geometrical dimension between the long axis and the short axis of the nanorod, results of the equivalent refractive index of the long axis direction and the short axis direction are different, and the anisotropic birefrigent effect is formed. Based on the Jones matrix, the feasibility of polarization converter is described. The polarization converter efficiency and polarization state of the structure are simulated and analyzed by the finite-difference time-domain method. And the variation characteristics of polarization converter transmittance are simulated under several nanorod with different heights and widths. In order to improve the contrast ratio and the transmission, the effective medium theory is used to design the metal grating for improving the transmission. According to the theory of optical thin film, we design the subwavelength metal grating with suitable duty cycle as the anti-reflection coating. The simulation results show that the structure can realize 90 rotation of linearly polarized light, the polarization converter efficiency is greater than 60% in a spectral range of 3.4-4.5 m and the contrast ratio is greater than 104 in a spectral range of 3-5 m. This structure can effectively realize the 90 polarization conversion in the spectral range of medium wave infrared and has the advantages of high conversion efficiency and high contrast ratio. In addition, the range of spectral of polarization conversion can be changed by adjusting the height and width of the nanorod. It can be applied to optical transmission control of optical network and optical information system, because of its excellent optical performance with the advantages of high polarization conversion efficiency and wide band in the mid-infrared waveband and low preparation difficulty.