A novel approach to design the subwavelength metal gratings for the polarizing beam splitters is presented. The method is based on effective medium theory （EMT） and thin film optics. The polarizing diffraction performance of subwavelength grating was analyzed by the rigorous couple-wave analysis （RCWA） for the wavelength of 650nm. The numerical calculus is used to analyze the effects of the zeroth diffraction efficiency caused by the grating period, the groove depth, the incident angle and the incident wavelength. The results show that subwavelength metal grating have the characteristics of high reflectivity like metal films for TE polarization and high transmission like dielectric films for TM polarization. Both transmission waves and reflected waves have high polarization extinction ratio and low insertion loss over a wide incident angle range （-30°θλ<800nm）. The optimal design agrees excellently well with the results simulated by RCWA method.