The invention discloses a single-phase reversible and oxidation-resistant hydrogen-storage high-entropy alloy and a preparation method thereof. A component expression of the high-entropy alloy is Ti0.2Zr0.2Hf0.2MoaNbb, wherein a is larger than or equal to 0 and smaller than or equal to 0.3, b is larger than or equal to 0.1 and smaller than or equal to 0.4, and the sum of a and b is equal to 4. Thepreparation method of the high-entropy alloy comprises the following steps of (1) raw material preparation, wherein various metal particles are weighed and prepared according to the atomic percent; (2) particle mixing, wherein the raw materials are screened and classified according to melting point characteristics of metals, and then the raw materials are mixed and put into a melting furnace crucible; and (3) alloy melting, wherein the metal particles are melted through a vacuum non-consumable arc-melting furnace. The high-entropy alloy has the advantages of being single in phase, single-phase reversible in phase before and after hydrogen absorption, high in oxidation resistance, high in speed for hydrogen absorbing and releasing and the like, and has a wide application prospect in the hydrogen energy field. According to the preparation method of the high-entropy alloy, preparation is conducted through the vacuum non-consumable arc-melting furnace, and therefore the preparation methodis simple, reliable, good in safety and high in economic value.