The invention discloses a composite diamond coating hard alloy cutter mold and a preparation method thereof. The surface layer of the hard alloy cutter mold is provided with composite coatings composed of boron-doped or non-boron-doped micron-crystal diamonds and nanocrystalline diamonds which are alternately arranged, and a layer of diamond-like coating DLC is deposited on the surface of each composite diamond coating. The preparation method comprises the steps of oil removal and degreasing, chemical micro-etching, plasma activation, plasma enhanced gaseous boronizing, plasma cleaning, diamond slurry ultrasonic grinding, nano-scale and micron-diamond seed crystal planting, and vapor-phase deposition of the diamond composite coatings. According to the method, boron atoms enter diamond crystal lattices by doping boron in the diamond deposition process, the internal stress of the diamond coatings is adjusted, and the generation of a stress sudden change region is avoided, so that the situation that the stress of each layer is not matched is avoided fundamentally, the stress distribution inside the composite coatings is effectively improved, the integrity of the micro-nano composite coatings is further improved, the substrate-film binding force is improved, and the comprehensive mechanical property of the composite coatings is improved.