The invention provides sulfur-doped cobalt oxide nanopowder, a preparation method thereof and application thereof to electrolytic water. A cobaltous dihydroxycarbonate precursor is synthesized by cobalt acetate under the alkaline condition and is annealed in the air to efficiently obtain a porous cobaltosic oxide nanosheet. Synthesis of the sulfur-doped cobalt oxide is realized by taking the porous cobaltosic oxide nanosheet as a raw material and taking an organic sulfur source compound as a reducing agent and a sulfur source at the same time. When the sulfur-doped cobalt oxide is applied to oxygen-producing electro-catalytic reaction, a good effect is achieved, the overpotential of oxygen production by catalytic water decomposition is reduced to be 0.36 V (relative standard hydrogen electrode), and the Tafel slope is low to 43 mV/dec.