In this thesis, we investigate the illumination system design of flash lens with asymmetric field light source. And we research the distribution of rays and observe the variation of light field by optical software, LightTools and CODE V. Besides, in order to use light power effectively and achieve the design targets, we design two lenses, which are symmetric lens and asymmetric lens. In the end of thesis, we analyze the distinctions, such as illumination, efficiency, tolerance and limitation between two designs.