The invention discloses molybdate Mg2.5VMoO8 used for an oxyen ion conductor and a preparation method thereof. The preparation method comprises the following steps: weighing the corresponding raw materials according to stoichiometric ratio of Mg2.5VMoO8, performing ball milling, pre-burning the material under high temperature and rapidly taking the material out, cooling the material, performing ball milling again, and finally performing cold isostatic pressing and sintering on the material to obtain Mg2.5VMoO8. The preparation method is simple, and the method is suitable for large-scale production. The synthesized Mg2.5VMoO8 has high ion conductivity under high temperature, high heat stability and chemical stability, and is an excellent oxyen ion conductor material.