Disclosed are an extremely-low temperature environment large-aperture reflecting optical telescope frost-prevention film system and a preparing method thereof. The structure of the frost-prevention film system is as follow: a transparent conductive film layer, an immediate insulating layer, a reflecting film layer and an outermost medium protection layer are successively disposed from a surface of a main mirror; and two edges of the transparent conductive film layer are provided with conductive electrodes. Steps are as follow: (1) preparation work, (2) a conductive film plating step, (3) an immediate insulating layer plating step, (4) a reflecting film plating step, (5) a plating step of the outermost medium protection layer, and (6) a plating step of the conductive electrodes. The extremely-low temperature environment large-aperture reflecting optical telescope frost-prevention film system and the preparing method thereof can be used in extremely-low temperature environment, and, through combination with mirror surface temperature rise control, the mirror surface is prevented from frosting. Reasonable design for the structure of the frost-prevention reflecting film system, and the extremely-low temperature environment large-aperture reflecting optical telescope frost-prevention film system has a frost-prevention function and also guarantees optical spectrum reflecting performance of the film system at the same time. When a film layer reflecting material also uses aluminum, in a 400nm-2000nm wave band scope, the average value of the optical spectrum reflectivity of the frost-prevention reflecting film system is better than 89%.