The invention discloses a WC hard alloy thin film. The WC hard alloy thin film comprises, on a substrate, a W bottom layer, a W-WC gradient layer and a WC top layer in sequence; the W bottom layer isa buffer layer; the W-WC gradient layer is a transition layer of which components gradually change; the WC top layer is a crystalline state WC thin film with an X-ray diffraction peak of (100) and a hexagonal phase structure, and W:C atomic percentage of the WC top layer is 51.8:48.2; and the microhardness of the WC hard alloy thin film is higher than 24 GPa, and the adhesive force to the substrate is higher than 35N. The invention further discloses a preparation method of the WC hard alloy thin film. The preparation method comprises the steps that a W thin film is deposited to be a bottom layer thin film on the substrate as a buffer layer by using a direct current magnetron sputtering method; then a W-WC gradient layer thin film is grown on the W bottom layer thin film at room temperature; and finally, after the W-WC gradient layer thin film is grown, the WC thin film is grown on the W-WC gradient layer thin film at the room temperature, and the WC thin film is a top layer thin film.According to the WC hard alloy thin film and the preparation method thereof, a W-WC gradient layer technology is adopted, and a W buffer layer technology and a plasma enhancement technology are combined, so that the quality of the WC thin film grown at the room temperature is effectively improved.