Growing concern about sulfur dioxide (SO2) emission from diesel exhaust causing a serious threat to the environment and human health necessitated enhanced desulfurization materials for dry desulfurization filters. MnO2 was modified with various alkali metal ions by impregnation to strengthen its SO2 capture capacity. The prepared materials were characterized by various technologies. Their SO2 capture capacities were measured by thermogravimetry, and the sulfate reaction mechanism was also evaluated. The results confirm that the SO2 capture performance of MnO2 could be significantly enhanced by doping with alkali metals. The prepared Li-doped MnO2 has good SO2 trap capacity with improved absorbance being 64% higher than that of MnO2. The grain model can be applied to explain the sulfate reaction mechanism between Li-doped MnO2 and SO2. The activation energy was calculated by the grain model equations.