Mordenite is highly active for the carbonylation of dimethyl ether reaction, and the regulation of its acid sites is an effective way to improve its catalytic performance. In this paper, a series of mordenite was subjected to the post-treatment of oxalic acid solution. The changes in the structure and acid sites were characterized by X-ray diffraction, N2 adsorption-desorption, scanning electron microscopy, NH3-temperature-programmed desorption and Py-infrared spectroscopy, respectively. The results show that the overall relative crystallinity and crystal phase of mordenite remain stable. However, the acid wash post-treatment can effectively remove parts of the acid sites through "etching" or complexation, especially those located in the twelve-membered ring channels. Meanwhile, it can create a certain amount of mesopores inside the particles, leading to an increase of the mesopore volume. The catalytic test results of dimethyl ether carbonylation reaction indicate that the sample treated with a suitable concentration of oxalic acid solution can effectively improve the dimethyl ether carbonylation activity. The dimethyl ether conversion significantly increases, despite the methyl acetate selectivity only slightly decreases. However, parts of the active sites responsible for the carbonylation reaction are removed when treated with high-concentration oxalic acid solution, resulting in a decrease in the catalytic activity. This research provides a simple and effective method for the regulation of the acid sites in mordenite and the improvement of its dimethyl ether carbonylation activity.