Solar simulator is an important application in non-imaging optics, its optical path is long because of the special optical system, and the collimating system accounts for nearly half of the whole optical path when the dual separation collimating lens are used. In order to achieve the miniaturization of solar simulator, the telephoto lens is used, and the telephoto lens has a short back focal length. When the dual separation collimating lens is replaced by the telephoto lens, the optical path of the collimating system can be shorter and the whole solar simulator optical path can be more compact. The collimating system with three- piece telephoto lens is designed, and the impact on the solar simulator technical specifications is studied through LightTools software simulation analysis. The simulation results show that although the solar simulator technical specifications change little. The simulator optical path becomes shorter, therefore the solar simulator can be miniaturized by using the telephoto lens as collimating optical system.