Catalyst of SO₃H group supported on mesoporous MCM-41 was prepared by direct-synthesis method. The composition of acid sites on catalyst surface was discussed, and the catalytic performance of o-xylene regioselective nitration was investigated under various technology conditions. The structure of MCM-41-SO₃H was characterized by the gravimetry of BaSO₄, transmission electronic microscopy(TEM) and N₂ adsorption/desorption. Results show that mesoporous structure of MCM-41 is maintained in the MCM-41-SO₃H system, there are Bronsted acid sites on catalyst surface with high BET surface area of 560 m²·g^-1. The optimum conditions of technology are as follows: calcination temperature of catalyst is 290 ℃, the reaction is 65 ℃ for 3 h with m_o-xylene/m_catalyst being 27 and n_{nitric acid}/n_o-xylene being 2.5 to get o-xylene conversion of 92.4%, mass percentage of 3,4-dimethylnitrobenzene reaching up to 83.3%.