The invention relates to a preparation method of a difluoro-sulfonyl imine salt, and belongs to the field of fine chemical engineering. The difluoro-sulfonyl imine salt is MN(SO2F)2, wherein M is Li, Na, K, Rb or Cs. The preparation method comprises the following steps: mixing FSO2X, a nitrogen source, an acid-binding agent and an aprotic solvent to obtain a mixture, wherein X is F, Cl, Br or I, the nitrogen source is anammonium salt or anhydrous NH3, the acid-binding agent is a lithium, sodium, potassium, rubidium or cesium salt, the mole ratio of NH3 or ammonium ion to FSO2X is 1:(2-10), and the mole ratio of NH3 or the ammonium ion to the acid-binding agent is 1:(3-10); heating to perform reflux reaction on the mixture for 5-8 hours; cooling to obtain a filtrate; and removing the solvent to obtain a crude product, and performing vacuum drying to obtain the difluoro-sulfonyl imine salt. The preparation method has easily available raw material, is safe and pollution-free, is high in yield, is easy to realize separation and purification, is simple in step, is not severe in requirement of operation equipment, and is suitable for industrial production.