Abstract: In order to achieve the illumination of maskless lithography by rectangular collimated flat-top laser beam and improve the illumination efficiency, a design method of the illumination system using double freeform surfaces shaping unit was proposed. Based on the optical path constant condition and Snell's refraction law, the surfaces equations in integral form of the freeform surfaces were derived. The integral equations were solved by numerical calculation. The illumination systems using two-lens shaping unit and single-lens shaping unit which included double freeform surfaces were designed and simulated by optical design software. The simulated results show that the uniformity is over 93% and the illumination efficiency is over 91%. It is illustrated that the two kinds of shaping unit could meet the requirements of the illumination of maskless lithography.