The invention discloses a method for preparing bi-(sulfonyl fluoride) imine and (fluorinated alkyl sulfonyl fluorine sulfonyl) imine alkali metal salt. The method comprises the following steps of: reacting sulfamide and thionyl chloride and chlorosulfonic acid to prepare bi-(sulfonyl chlorine) imine and (fluorinated alkyl sulfonyl chlorine sulfonyl) imine, reacting with antimony trifluoride and potassium carbonate (rubidium or caesium) to obtain corresponding high-purity bi-(sulfonyl fluoride) imine potassium (rubidium or caesium) salt or (fluorinated alkyl sulfonyl fluorine sulfonyl) imine potassium (rubidium or caesium) salt, performing double decomposition exchange reaction using the potassium (rubidium or caesium) salt with the lithium perchlorate (or sodium) or lithium tetrafluoroborate (or sodium) in the aprotic polar solvent to obtain corresponding lithium (or sodium) salt with high purity. The method in the invention has the characteristics of simple operating steps, easy separation and extraction of output, high purity and yield, no environmental pollution, and the like, and is suitable for mass industrial production.