Abstract: p-acdophenyl trimethyl ammonium iodide was synthesized with p-dimethyamino acetophenone and methyl iodide in ethyl acetate solution. The structure was characterized by infrared spectroscopy and nuclear magnetic resonance spectroscopy. The effect of reaction conditions on the yield was investigated. The results showed that in ethyl acetate solution, when the methyl iodide was used by an excess of 50%, the resulting product will precipitate out of the solution. When the reaction time lasts for 4 hours, the yield can reach 85%.