The invention discloses a preparation method of low-salt heavy sodium carbonate. The method comprises the steps as follows: limestone calcination and lime milk preparation, compression of CO2 gas, preparation and refinement of brine, ammonia absorption of brine and ammonia distillation of mother liquor, carbonation of ammoniacal brine, sodium bicarbonate filtration, sodium bicarbonate centrifugation, sodium bicarbonate calcination, preparation of monohydrate sodium carbonate, preparation of heavy sodium carbonate, sodium carbonate cooling and finished product packaging. According to the preparation method of heavy sodium carbonate, a sodium bicarbonate suspension is centrifuged after being filtered, the salinity of wet sodium bicarbonate is reduced, besides, a liquid-phase hydration deviceis omitted on the basis of a traditional solid-phase hydration method, type I low-salt and high-quality heavy sodium carbonate is produced directly, the construction cost and the operation cost are reduced, energy consumption is reduced, and the method is environmentally friendly.