High performance infrared polarizer with broad band is required for various infrared applications. The conventional infrared polarizer, based on the birefringence effect of natural crystal, is cost-consuming in fabrication and can hardly be integrated into micro-optical systems due to its large bulk. In this paper, an infrared polarizer is proposed in the spectrum from 3 to 19 μm based on sub-wavelength metal wire grid. The dependence of the performance on some key parameters, including metal materials, geometrical parameters, has been deeply investigated by using the Finite-Difference Time-Domain (FDTD) method. The results show that Au wire-grids have a higher transmittance for the Transverse Magnetic（TM） mode light than that of other metal materials, and both the grid period and the grid thickness have important impact on the performance. Based on these observations, a polarizer has been designed by choosing the optimal value of related parameters. Numerical simulation suggests that the designed infrared wire grid polarizer have advantages of broad band, high TM polarization transmission efficiencies and high extinction ratios. The transmission efficiencies of TM polarization are larger than 59.3%, and the extinction ratios range from 28.6 to 44.6 dB in that range of the spectrum.