The invention discloses a preparation method of food-grade magnesium chloride hexahydrate. The preparation method comprises the steps as follows: selection of bromine producing mother liquor, bleaching, desulphuration, filtration, evaporation, settlement and sheet preparation. The bromine producing mother liquor is added to a reaction kettle, steam heating devices are arranged on the periphery ofthe lower part of the reaction kettle and introduce steam to heat the reaction kettle, vacuum pumping equipment is externally connected with the upper part of the reaction kettle for vacuum pumping pressure reduction, a feed liquid is obtained through vacuum evaporation, and ultrasonic high-frequency vibration is performed to avoid condensation during evaporation. With the adoption of the preparation method of food-grade magnesium chloride hexahydrate, the vacuum degree is increased and the boiling point final temperature is decreased by use of vacuum pressure reduction, ultrasonic high-frequency vibration is performed to avoid condensation, evaporation is accelerated, and the working efficiency is improved.