Method of preparing heteropoly acid catalyst

The present invention relates to a method of preparing a heteropoly acid catalyst used for the production of methacrylic acid by gas phase oxidation of methacrolein, more precisely a method of preparing a heteropoly acid catalyst comprising the steps of preparing a slurry by adding metal precursors and ammonium salt to protonic acid Keggin-type heteropoly acid aqueous solution and stirring thereof; and drying, molding and firing the slurry to give a catalyst. The present invention provides a method of preparing a heteropoly acid catalyst exhibiting high methacrolein conversion rate and methacrylic acid selectivity without pre-firing process by using high purity protonic acid Keggin-type heteropoly acid and ammonium salt.