A method for preparing ultrafine powder of tungsten carbide using ultrafine tungsten powder and carbon black as raw materials is provided. The following steps are included: (1) passivation of the ultrafine tungsten powder: passivating the ultrafine tungsten powder under pure carbon dioxide; (2) carbon addition: mixing the ultrafine tungsten powder with carbon black powder after applying cooling water and inert gases; (3) carbonization: synthesizing the bulk tungsten carbide powder at high temperature in a carbonizing stove; (4) crushing and sieving: crushing the bulk tungsten carbide powder, cooling and sieving to obtain the ultrafine powder of tungsten carbide.