(NH₄)₅H₆PV₈Mo₄O₄₀ is an improved bi-functional catalyst that has been used for oxidizing starch by hydrogen peroxide. The effects of oxidation with hydrogen peroxide on the carboxyl content were studied systematically to produce high polyhydroxycarboxylic acids even when using very low amounts of the catalyst. (NH₄)₅H₆PV₈Mo₄O₄₀ exhibited the best catalytic performance with a carboxylic content of 0.648 mol per 100 g, which could be attributed to the synergy of its Brønsted acidity and strong efficient oxidant center. This catalyst can perform controllable starch oxidation and can be recycled for reuse.