Polarization compensation is one of the key techniques of a polarization encoding quantum key distribution (QKD) system. This paper introduces the principles of polarization compensation by polarization controllers or a bidirectional optical structure. Some typical polarization compensation schemes suitable for polarization encoding QKD system based on optical fibers are sketched, such as an all-fiber polarization compensation scheme based on a bidirectional optical structure, an interrupting scheme of polarization compensation, a time division multiplexed (TMD) polarization compensation scheme and a wavelength division multiplexed (WDM) polarization compensation scheme.