The invention discloses a preparation method of high-entropy oxide ceramic. The method comprises the following steps that 1, according to metal atom mole ratio being 1:1:1:1 or 1:1:1:1:1, four or fiveor less kinds of metal oxide powder are weighed; the oxide powder includes MgO, ZnO, NiO, CuO, CaO, CoO, ZrO2, CeO2, Al2O3, Gd2O3, La2O3, Er2O3, Y2O3, Fe2O3, Co3O4 and CaCO3; 2, powder weighed in step 1 is subjected to ball milling, drying and granulation; 3, granulated powder is pressed and molded to form a green body; 4, the green body obtained in step 3 is subjected to thermal treatment; 5, the green body obtained after thermal treatment in step 4 is heated to the preset temperature, an electric field of the preset electric field intensity is applied to a sample, after flash-burning appears, a power source is converted from the constant voltage state to the constant current state, the temperature is kept constant for 1-60 min under the preset current density, and then quenching is performed to obtain the needed high-entropy oxide ceramic. The flash-burning is adopted for preparing the high-entropy oxide ceramic, the sintering temperature is remarkably lowered, and the sintering time is remarkably shortened.