The invention relates to the field of chemical synthesizing, in particular to a preparation method of imidodisulfuryl fluoride lithium salt. The preparation method includes the steps of firstly, allowing chlorosulfonic acid and chlorosulfonyl isocyanate to react in the presence of catalyst to obtain dichlorosulfimide; secondly, allowing dichlorosulfimide and hydrogen fluoride to react in the presence of catalyst to obtain imidodisulfuryl fluoride; thirdly, allowing imidodisulfuryl fluoride to react with compound containing lithium to obtain the imidodisulfuryl fluoride lithium salt. The preparation method has the advantages that the chlorosulfonic acid and chlorosulfonyl isocyanate are used as the raw materials in the first-step reaction, the generation of waste gases such as SO2 and HCl is avoided, and environment protection requirements are satisfied.