Finding out the mechanism how copper improves the catalytic performance is important to design efficient heteropolyoxometalate catalysts for clean production of methacrylic acid. In this work, copper containing heteropolyoxometalate catalysts (CsCuxH3-2xPMo11 VO40) were synthesized, characterized and used to catalyze the oxidation of methacrolein (MAL) to methacrylic acid. The FT-IR, TG-DTA and XPS results showed that copper was in the forms of Cu2+ and Cu+ and complexes with 4 water molecules in the secondary structure of the catalysts at room temperature. XRD and BET analysis showed that the addition of copper could increase the crystalline size of catalysts, prevent the accumulation of catalyst crystal and make catalysts to be a kind of mesoporous materials with narrow pore width distribution (around 4nm), which is positive for the catalytic performance. The catalytic tests showed that the addition of copper can obviously improve the catalytic performance of catalysts. Copper can get an electron from Keggin structure to improve the oxidation ability of active sites during the oxidation of MAL and give an electron to Keggin structure to make the reduced active sites to be re-oxidized easily. This paper gives a clear picture of copper states in the heteropolyoxometalate catalysts and the mechanism how copper improves the performance of heteropolyoxometalate catalysts s for oxidation reactions. (C) 2017 Elsevier B.V. All rights reserved.