Tetramethylammonium hydrogen carbonate was synthesized directly from dimethyl carbonate and trimethylamine in water. The effects of materials molar ratio, reaction temperature and time on the yield of product were investigated. The optimum reaction conditions were listed as follows: dimethyl carbonate and trimethylamine molar ratio was 1:1.2, reaction temperature was 105℃, and reaction time was 5h, yielding product up to 93.0%.