The present invention relates to a hydrothermal preparation method of cobaltosic oxide nanocube and an application of the cobaltosic oxide nanocube in electrode slice preparation. The hydrothermal preparation method of the cobaltosic oxide nanocube comprises the steps of weighing the cobalt salt, the alkali and a surface active agent separately to dissolve in the deionized water; weighing the cobalt salt solution and the alkali solution which have the equal volume to carry out the constant temperature water bath separately, stirring and heating uniformly, and then adding the alkali solution in the cobalt salt solution, after the alkali solution and the cobalt salt solution react completely, adding the surface active agent solution to obtain the mixed solution; transferring the mixed solution in a sealed tank to carry out the hydrothermal processing to obtain the black precipitates; washing the black precipitates with the deionized water and the absolute ethyl alcohol orderly, drying at the air atmosphere to obtain the cobaltosic oxide nanocube; mixing the cobaltosic oxide nanocube with a conductive agent and a binder and stirring to obtain the slurry; scraping and coating the slurry on a preprocessed foamed nickel uniformly, drying and compressing to obtain an electrode slice. The preparation method of the present invention needs simpler equipment conditions, is convenient to operate, is low in preparation cost, and is suitable for the factory large-scale production.