The invention provides a hard alloy part with a diamond coating. The hard alloy part comprises a hard alloy base body, a carborundum layer, a carborundum-diamond gradient composite coating and the diamond coating, wherein the carborundum layer, the carborundum-diamond gradient composite coating and the diamond coating are sequentially arranged on the base body; in the carborundum-diamond gradient composite coating, the content of the carborundum is gradually reduced, and the content of diamond is gradually increased in the thickness direction from the carborundum layer to the diamond coating; the carborundum layer and the carborundum-diamond gradient composite coating are deposited on the hard alloy base body firstly to serve as a middle layer, then, the diamond coating is deposited, the component of the whole coating on the surface of the base body shows gradient changes from the base body to the top layer, increasing of a new interface does not exist, the heat expansion coefficient shows the gradient changes, and therefore heat stress of a diamond thin film can be effectively reduced, and the thin film combining force is improved. The invention further provides a preparing method of the hard alloy part with the diamond coating.