The invention relates to a heteropolyacid catalyst and a preparation method of the catalyst. The heteropolyacid catalyst includes phosphorus, molybdenum, vanadium, copper and arsenic, all of which are taken as necessary active components, and a composition of cuprous oxide and phosphoric acid is taken as all or a portion of copper-containing material of the heteropolyacid catalyst. The preparation method of the heteropolyacid catalyst comprises the following steps: mixing compounds including the corresponding elements with water, heating a mixture to obtain a settled solution, drying the obtained settled solution while controlling the average particle diameter of the dried catalyst powder to be 0.2-10 microns, and finally calcining the catalyst powder in segments to obtain the heteropolyacid catalyst, wherein the number of single-molecule water of crystallization of the heteropolyacid catalyst is 1-5. When the heteropolyacid catalyst disclosed by the invention is applied to preparation reaction of methacrylic acid by utilizing methylacrolein, the conversion rate of methylacrolein and the selectivity of methacrylic acid are relatively high.