The production of isophthalic acid (IPA) from the oxidation of m-xylene (MX) by air is catalyzed by H3PW12O40 (HPW) loaded on carbon and cobalt. We used H2O2 solution to oxidize the carbon to improve the catalytic activity of HPW@C catalyst. Experiments reveal that the best carbon sample is obtained by calcining the carbon at 700 °C for 4 h after being impregnated in the 3.75% H2O2 solution at 40 °C for 7 h. The surface characterization displays that the H2O2 modification leads to an increase in the acidic groups and a reduction in the basic groups on the carbon surface. The catalytic capability of the HPW@C catalyst depends on its surface chemical characteristics and physical property. The acidic groups play a more important part than the physical property. The MX conversion after 180 min reaction acquired by the HPW@C catalysts prepared from the activated carbon modified in the best condition is 3.81% over that obtained by the HPW@C catalysts prepared from the original carbon. The IPA produced by the former is 46.2% over that produced by the latter.