The invention provides a preparation method of pure single-phase ternary carbide Co3W3C and belongs to the field of hard alloy material preparation. Cobalt powder, tungsten powder and carbon powder serve as raw materials and are blended and subjected to intermittent ball milling together with hard alloy balls of different diameters and absolute ethyl alcohol, then cold pressing and a chemical reaction are conducted, and a green body containing the pure single-phase ternary carbide Co3W3C powder is obtained; then intermittent ball milling and crushing are conducted, sieving with a 400-600-mesh mesh screen is conducted, and the Co3W3C powder which is pure in physical phase and uniform in granularity is obtained; the Co3W3C powder is arranged in a die, subjected to cold pressing and forming, placed into a discharging plasma sintering equipment for sintering and cooled to the room temperature, and a Co3W3C block material containing a single physical phase can be obtained. A product obtained through the method is a pure single-phase substance.