The splitting character that the reference wavelength, incident angle and the number of thin-films change with wavelength is analysised based on Macnielle type thin-film prism by using TFCALC software, which will contribute to the optimization and deposition technology of this kind of prism. We proposed a optimizing design of a polarizing beamsplitter whose substrate is LaK2 and the thin film material is ZrO2 and SiO2.