The invention discloses a heteropoly acid ammonium salt catalyst and a preparation method thereof. The catalyst comprises an active component, an assistant and a carrier. The active component is heteropoly acid ammonium salt, the assistant is nickel oxide, and the carrier is silicon oxide. The catalyst provided by the invention is especially suitable for the reaction of preparing ethylene by dehydration with low concentration ethanol aqueous solution as the raw material. The catalyst not only has the characteristics of high low temperature activity, good selectivity, and strong carbon deposition resistant ability, but also has good stability.