When used at oblique angle incidence, optical thin films exhibit strong polarization effects. This phenomenon will make the optical performance worsen in most application cases. But the design of optical thin film system to control the polarization effect is very difficult. The physical mechanism of the polarization effect is analyzed first, and then a near-infrared two-waveband nonpolarizing beam splitter at 1300~1330 and 1535~1565 nm spectral ranges with the splitting ratio of 11 is designed. The initial thin film stack is formed with a four-layer set and an equivalent layer using three all dielectric materials. Then it is optimized in multiple steps by simplex and conjugate gradient algorithms. According to the results, in the working spectrum range with the incident angle of 45°, it maintains a very low polarization ratio whether reflectance or transmittance induces phase changes.