The present document relates to a method for preparing trimethylammoniumethionyl diethyl ester iodide, comprising : reacting sodium O, O-diethylthiophosphate with 2-dimethylaminohaloethane to obtain dimethylaminoethionyl diethyl ester; reacting dimethylaminoethionyl diethyl ester with methyl iodide to obtain trimethylammoniumethionyl diethyl ester iodide. The method for preparing trimethylammoniumethionyl diethyl ester iodide provided herein is simple, easy-to-implement, low-cost, high-yield, and suitable for large-scale industrial production.