Abstract: According to the validation requirements of the field test, a large aperture optical system with precisely zooming was designed, in order to provide target imaging simulation from near field to far field to test and evaluate the system performance. The system has two parts: zoom system and projection system. Based on the requirements of beam aperture and image size on the unit to be tested, and with the precondition of pupil connection and aperture matching principle, the optical parameters of projection system and zoom system were calculated and the image quality was optimized. The zoom system works between 8 m ~12 m in far-infrared band with a 16x zoom ratio. The large-aperture projection system has a 300mm aperture. Simulation experiment results show that, the image position is stable and the modulation transfer function (MTF) curve of each focal length is close to the diffraction limit during zoom process, which could meet the test requirement exactly.