The invention discloses a synthesis method for a bis(chlorosulfonyl)imide salt, wherein the method comprises the following steps: (one) under the protection of nitrogen, dissolving bis(chlorosulfonyl)imide in a nonprotic organic solvent, and diluting to obtain a bis(chlorosulfonyl)imide organic solution, wherein in the diluting process, the temperature is controlled in 0 DEG C-15 DEG C, and the water content of the nonprotic organic solvent is controlled in 25-100 ppm; (two) adding an alkali metal salt into the organic solution, slowly heating to 80 DEG C-140 DEG C while stirring, controlling the water content of the alkali metal salt in 50-100 ppm and the molar ratio of bis(chlorosulfonyl)imide to the alkali metal salt to (1:1)-(1:1.1), and thus obtaining an organic solution after the reaction; (three) under the protection of nitrogen, filtering the organic solution to obtain a filtrate, and concentrating to obtain a concentrated solution; and (four) crystallizing the concentrated solution, filtering, drying, and thus obtaining the high-purity bis(chlorosulfonyl)imide salt. The method is simple and feasible to implement, and the preparation cost is low. The purity and the yield of the product are higher, and the water content is extremely low.