In order to improve the extender ratio of parallelism beam splitting, a new design scheme of bireflective parallel distensible beam splitting prism is proposed. By using prism expanded beam principle and Fresnel formula, some factors, such as structure angle and refractive index, which influence the extender ratio and transmissivity are analyzed. The effects of increasing shear difference on the basis of high transmissivity and the optimal design of structure are properly realized in the new scheme. The scheme has the deflecting function which could decrease the linear length of rays path building and save space of flat roof in experiment. The testing results show that the transmissivity and the shear difference of this new polarization prism are perfect and the experimental values are in consistent with the theoretical analysis. It is an ideal parallel beam splitting polarization device and has important practical merits.