In order to realize free space quantum key distribution, an optical system with the property of retaining the polarization state is customized to reduce the quantum bit error rate, and the mechanism and application of phase shift in polarized optical system is investigated. Firstly, based on the principle of matrix optics, the relationship among azimuth angle, phase shift and extinction ratio of polarized light is presented. Then the principle of phase shift added linearly is validated in the theorem and experiments. Based on the principle of phase shift added linearly, a set of polarization keeping system is designed and validated in the theorem and experiments，and the principle is applied to design optical systems for quantum communication successfully. Experimental results indicate that the extinction ratio of polarized light of optical systems for quantum communication exceed 500:1. It can satisfy the system requirements of quantum bit error rate for quantum communication.