The invention provides a prereduction method of a complete methanation catalyst. The prereduction method comprises the following steps: (1) a physical dehydration step; (2) a reduction step; and (3) a passivation step, wherein the reduction step includes reduction treatment at a low-temperature stage and a high-temperature stage. The invention further provides the complete methanation catalyst at a prereduction state prepared according to the method. Spinel nickel in the complete methanation catalyst is reduced by virtue of the heating reduction of the two stages. The complete methanation catalyst at the prereduction state prepared according to the method is high in heat stability and excellent in catalyst reaction property.