The optical system of a large moon simulator with the diameter of 2500 mm is designed. By controlling the LED light of different bands and combining it into multiple group elements and lamp arrays, the simulation of the moon spectrum and the transition of different moon phase in the spectral bands from 355 nm to 1000 nm is achieved. Thus the geometric characteristics and radiation characteristics of the particular “phase” are simulated. We introduce the composition and working principle of large moon simulator, and simulate and analyze the indicators including radiance and irradiation non-uniformity of different phases in the moon simulator by the LightTools software. The results show that the radiance is 32 W/(sr·m2) when the moon is full, the illumination non-uniformity is 14.7%, and the brightness non-uniformity is 17.2% on the luminous surface of different phases of the moon. These parameters can meet the design requirements.