The invention relates to a preparation method for a mixed oxide, in particular to a preparation method for an iron-nickel-chromium mixed oxide catalyst. The method achieves the purpose of synthesis of the mixed oxide. The method comprises the steps that 1, iron salt, nickel salt and chromium salt are mixed and dissolved, and a surface active agent is added for stirring; 2, a precipitating agent is added into the solution to adjust the PH value; 3, crystallization is performed at a certain temperature, and precipitation, filtering and drying are performed; 4, mixed oxide powder is formed to obtain the catalyst. In the mixed oxide synthesis process, a precipitation hydrothermal method is adopted, the proper PH value needed by precipitation is found, the synthesis method is simple and easy to implement, the repeatability is good, and industrial production is easy to achieve.