Abstract: Under large-angle oblique incidence, the optical thin films exhibit strong polarization effects. The reflectance and transmittance for both S- and P-components will exhibit some polarization deviations. The reflection-and transmission-induced phase retardances will exhibit the same behavior. After the analysis of the physical essence within the polarization effects, a near-infrared nonpolarizing beam splitter with a reflectance of 50% in 1 500-1 600 nm spectral range was designed. It adopted a thin film stack with approximately equal equivalent admittances for both S- and P-components with the aid of simplex optimization algorithm. According to the results, in the working range of spectrum with the incident angle of 45, the splitter maintains a very low polarization ratio, and both the reflectance( transmittance) curve deviation and the reflection (transmission)-induced phase change are small.