The invention discloses a preparation method of difluorophosphoric acid alkali metal salt. In the method, the difluorophosphoric acid alkali metal salt is produced through a reaction of hexafluorophosphates of alkali metals and corresponding phosphates of the alkali metals in a solid phase or in a non-aqueous solvent, wherein the reaction equation is MPF6+M3PO4=2MPO2F2+2MF (M=Li, Na, K, Rb, Cs), the molar ratio of MPF6 to M3PO4 is 0.8-1.5:1, preferably 1.1-1.2:1, namely, the hexafluorophosphates of alkali metals is excessive properly to ensure reaction of the phosphates to be carried completely. The method can be used for preparing the difluorophosphoric acid alkali metal salt under safe situations, thereby improving safety of the preparation process and ensuring safety of preparation operators. The difluorophosphoric acid alkali metal salt is produced through a reaction of the hexafluorophosphates and the phosphates of the alkali metals, and the target product can be produced whether the reaction is carried out in solid phases or liquid phases under mild conditions without dangerous gas raw materials and generation of acidic gas or water which may damage performances of an electrolyte.