It is liquid crystal on silicon (LCOS) micro- projector′s characteristic that an image is formed by the reflected light from the LCOS′ surface, and the illuminating beam angle of divergence needs to be small, the uniformity error of illuminating should be fairly well. According to these requirements, an illuminating system on LCOS is designed, the projection device consists of a parabolic reflector, fly′ s- eye lens, a polarizing beam splitter, condenser lens and so on. The parameters of the parabolic reflector and the fly′s-eye lens are analyzed in detail. It includes the beam angle of divergence, the total length for the light source and the intended square to be uniform illuminated. Based on the theory of fly′ s eye′ s inform illuminating and the relationship between parameters of the parabolic reflector and the beam angle of divergence. The design requirements are that the beam angle of divergence should be smaller than 10°, the total length of the light source should be 220 mm, the area to be illuminated is 20 mm × 20 mm. Finally collimating lens is designed to collimate the rays on the LCOS, the illuminance should be as large as 0.35 lx/m2. And it is demanded that the uniformity error on the surface of LCOS should be limited in 15% . Simulation results are acheived by Lighttools. Based on the line charts and the grid charts, it has satisfied the design requirements.