As an important chemical raw material, methacrylic acid (MAA) is often used in the production of coatings, adhesives, ion exchange resins, etc. Among the production routes of MAA, the oxidation route of isobutene is a research hotspot. In this route, the first step is to produce methacrolein (MAL) by oxidation of isobutene, and then to produce MAA by catalytic oxidation of MAL with heteropoly acid catalyst. Keggin type heteropoly acid catalyst has been successfully applied in this route. In this work, phosphomolybdovanadic acid was took as an example to analyze the adsorption and reaction process of MAL on heteropoly acid catalyst. The results of in-situ FT-IR showed that the ester intermediate adsorbed on the bridging oxygen of heteropoly acid catalyst was important. After MAL adsorption, the ester intermediate was formed and further oxidized to carboxyl intermediate or carbon oxides. The mechanism of catalytic oxidation of MAL over heteropolyacid catalysts was further proposed based on the above conclusions. The reaction process was analyzed at different temperatures. The characteristic peak of vanadium oxide compound in the secondary structure of the catalyst was observed when the reaction temperature rose to 310℃, and the peak was enhanced with the increase of temperature. Referring the results of catalyst evaluation, the conversion of the reaction increased significantly when the vanadium oxide compound transferred to the second structure, indicating that it was the active site in the reaction. It was considered that the migration of vanadium oxide compound from primary structure to secondary structure was the key reason for the significant improvement of catalytic performance. By reduction with H2, the migration of vanadium oxide compound was promoted, and the conversion of MAL and the yield of MAA were further improved. After reduction at 250℃ for 3 h, the catalyst had the highest activity, and the yield of MAA increased from 17.3% to 27.2%.