The invention discloses a Co2O3 nanosheet and a preparation method thereof. The preparation method comprises the following steps: dissolving cobalt sulfate, cobalt chloride, boric acid, trisodium citrate and o-sulfobenzimide in distilled water to obtain a plating solution while controlling the addition of cobalt sulfate to 100-200g, the addition of cobalt chloride to 80-120g, the addition of boric acid to 8-15g, the addition of trisodium citrate to 100-200g and the addition of o-sulfobenzimide to 0.2-0.8g in each liter of distilled water; adopting ITO conductive glass as a substrate, and electroplating the substrate with the plating solution to form a metal cobalt coating; and oxidizing the substrate containing the metal cobalt coating by an oxidation process to obtain a Co2O3 nanosheet. The Co2O3 nanosheet is prepared by the preparation method. In the invention, the raw material cost is low, and the preparation technology is easily industrialized and has relatively great practical value; and moreover, the obtained product has the advantages of high yield, high purity, uniform morphology, narrow size distribution and great length-diameter ratio.