The invention discloses a heteropoly acid ammonium salt catalyst and a preparation method thereof. The catalyst adopts alumina as the carrier, takes heteropoly acid ammonium salt as the active component, and employs boron oxide as the assistant. The preparation method consists of: adding the alumina carrier into an assistant-containing organic acid water solution, conducting evaporation to dryness, then adding the product into an ammonium-containing alkaline solution or contacting it with ammonia gas, performing drying, then adding the obtained product into a heteropoly acid aqueous solution, performing evaporation to dryness, and then carrying out drying and roasting so as to obtain the catalyst. The catalyst provided by the invention has high activity, selectivity and carbon deposition resistance, and also improves the stability of the catalyst. The catalyst provided by the invention is particularly suitable for the reaction of preparing ethylene by dehydration with low concentration ethanol as the raw material.