In this paper,the depolarization mechanism of a monochromatic light crystalline quartz depolarizer is studied. The depolarization degree, which is a physical Paramter that shows the depolarization character of a depolarizer, is defined. The depolarization degree is measured in the spectrum region of 440￣～ 920 nm.The results show that if the structural angle of the depolarizer is suitable or the size of the incident ray is suitably controlled,the best depolarization can be achieved for a linearly polarized light.