The invention belongs to the field of hard alloy preparation, and in particular relates to an ultra-fine crystal gradient alloy with a rich-cobalt surface and a preparation method of the alloy. According to the ultra-fine crystal gradient alloy with the rich-cobalt surface, a gradient layer with the rich-cobalt surface, which is 10-40mu m thick, is formed by taking WC (Wolfram Carbide) and cubic phase carbonitride as a core part hard phase and cobalt as the binding phase, wherein the average size of the crystal particles of WC in the hard phase and the gradient layer is 0.2-0.4mu m, the content of cobalt in the gradient layer with the rich-cobalt surface is 1.2-2 times of the standard content of cobalt of hard alloys, and the gradient layer with the rich-cobalt surface is free of the cubic phase carbonitride. The preparation method comprises the following steps: firstly, preparing the materials, wet-grinding and pressing into a material blank, and supplying an inert gas with the pressure of 5-10MPa before reaching a liquid phase sintering temperature, wherein the liquid phase sintering temperature is 1,350-1,500 DEG C. By charging the gas with certain pressure in the liquid phase sintering period, the ultra-fine crystal gradient alloy with the rich-cobalt surface is prepared, the WC crystal grains are refined, a rich-cobalt surface layer of certain thickness is obtained, the sintering period is shortened, and the production cost is lowered.