Cationic etherifying agent OMTMA is a key reactant for preparing cationic flocculant, which directly influenced the flocculation effect of final synthetic flocculant. The influence factors on the preparation of cationic etherifying agent OMTMA was studied, and the optimal preparation conditions of cationic etherifying agent OMTMA is as follows: molar ratio of amine hydrochloride to Trimethylamine 2∶1, molar ratio of trimethylamine to epichlorohydrin 1∶0.8, reaction temperature 40 ℃, reaction time 2.5 h. OMTMA cationic etherification agent prepared is excellent on the synthesis of modified flocculant, which has a removal rate of 96.6%.