An extra constant difference between s and p component is produced after the light gets through the reflector based on total reflection,which lead to the change of the polarization state as a result. A novel design method of the polarization-preserving films for rectangular prism based on high-reflection films is proposed after deep analysis on its working mechanism and the geometry of the prism.The polarization state is kept invariant by plating the designed films on the two planes of the prism.The method has been theoretically and experimentally proved to be feasible.It is also easy to realize commercial application with the mature fabrication process of the high-reflection films. What′s more,the polarization-preserving characteristic of the method works well with little incidence error.