The invention discloses a preparation method for preparing anhydrous betaine through chemical synthesis. High-purity anhydrous betaine of which the purity is over 99.9 percent is produced by performing procedures including neutralizing, substituting, concentrating, primary desalting, secondary desalting, evaporating, crystalizing, centrifuging, drying and the like on chloroacetic acid, sodium carbonate and trimethylamine serving as major raw materials through a process for combining electrodialysis desalting, evaporative desalting and a boiling dryer. In the process, the original methanol dehydration and double-core dryer drying equipment is eliminated, and obtained metahydrate betaine is directly dried by using a boiling drier, so that cost on each ton can be saved by about 1,200 yuan, and the yield is increased by 3 tons per day.