The invention discloses a synthesis method of cationic monomer dimethyl diallyl ammonium chloride, belongs to the technical field of organic chemical industry, and particularly relates to synthesis of dimethyl diallyl ammonium chloride. The synthesis method comprises the following steps: firstly, performing distillation on a 30-40% dimethylamine solution at low-temperature, and absorbing by using a dimethylamine solution with the same concentration and in a low-temperature state to obtain a concentrated dimethylamine solution with the mass concentration of 55-65%; secondly, at 0-10 DEG C, slowly adding 1/8-1/ 6 of chloropropene into the concentrated aqueous dimethylamine solution, and performing early tertiary amination reaction; thirdly, adding the rest chloropropene and a sodium hydroxide solution with the mass concentration of 45-55% dropwise, and performing neutralization reaction and the early tertiary amination reaction; finally, performing quaternization reaction, and purifying to obtain the cationic monomer dimethyl diallyl ammonium chloride. According to the synthesis method, the distillation temperature is relatively low and the negative pressure environment is not required, so that the operation is safer, the reaction speed is high, few intermediate products are generated, and the production capacity of a device can be improved.