The invention belongs to the chemical catalyst technology field, and particularly, relates to an oxide catalyst for converting ethanol to prepare isobutene and a preparation method thereof. The catalyst is a ZrO2-ZnO-Sc2O3 mixed oxide catalyst and is obtained by a hydrothermal method. The preparation process includes the steps of obtaining a mixed hydroxide by a constant-pH co-precipitation method and then carrying out hydrothermal treatment and high-temperature calcination and the like. The catalyst can be used for a reaction of converting ethanol to prepare isobutene, and achieves higher isobutene yield and reaction stability.