Novel method for synthesizing quaternary ammonium salts

The invention discloses a method for synthesizing quaternary ammonium salt, which is characterized in that methyl-containing quaternary ammonium salts are prepared through the reaction of dimethyl carbonate, tertiary amine and inorganic acid. The method specifically comprises the steps: putting tertiary amine in proportion in a reactor; pumping a corresponding amount of mixture of inorganic acid, polar solvent and ionic liquid catalyst in appropriate controlled flow into the reactor; stirring for reaction at an appropriate temperature in nitrogen atmosphere; putting vacuum-dried products after reaction is completed in the reactor; blowing and washing the reactor with nitrogen; pumping the mixture of dimethyl carbonate and polar solvent in appropriate controlled flow into the reactor for hermetic reaction; performing reaction under the pressure between 0.5 and 3 Mpa at reaction temperature between 100 and 200 DEG C; stirring till reaction is completed; and obtaining reaction products which are the methyl-containing quaternary ammonium salts. The method greatly improves the utilization rate of catalysts and is small in the amount of catalysts, and the reaction yield and purity of the method are over 99 percent.