The invention provides an ultra-smooth dual-band reflector and a preparation method thereof, wherein the ultra-smooth dual-band reflector includes a substrate and a reflective film plated on the substrate, and the reflective film is composed of alternately laminated platinum films and non-metallic films , the metal film includes a chromium film and a platinum film, the chromium film is plated on the substrate, the platinum film and the non-metal film are alternately laminated and plated on the chromium film, and the top layer and the bottom layer in the laminated structure formed by the platinum film and the non-metal film are both It is a chromium film, the thickness of the chromium film is 1-3nm, the thickness of each platinum film is 5-30nm, the thickness of each non-metallic film is 1-100nm, and the total thickness of the reflective film is 100-300nm. The invention can ensure low high-frequency roughness and intermediate-frequency roughness, and at the same time, has high reflectivity for imaging bands, and has strong resistance to space environment radiation, good space adaptability and stable performance.