A preparation method of composite oxide is characterized by comprising the steps as follows: oxide powder or an oxide tablet and a metal current collector are composited to be used as a negative pole, graphite is used as a positive pole, molten alkali metal halide or alkaline-earth metal halide is used as an electrolyte, or molten alkali metal halide or alkaline-earth metal halide with excessive corresponding alkali metal oxide or alkaline-earth metal oxide added is used as the electrolyte, electrolysis is performed in air or in the protective atmosphere at the electrolysis temperature of 400-900 DEG C and under the electrolysis voltage of 1.4-3.2 V for 1-5 h, a product is taken out after being cooled to the normal temperature and then is washed in distilled water or an organic solvent, and a mixture of the alkali metal oxide or the alkaline-earth metal oxide and reduced oxide is obtained after vacuum drying; and the mixture is cleaned, is subjected to vacuum drying and is fired at 600-1,200 DEG C for 1-3 h in the air, vacuum or the inert atmosphere, and the composite oxide is obtained. With the preparation method of the composite oxide, the size of particles is controllable, and the batch stability and consistency are guaranteed.