The present invention relates to a novel method for preparing lithium bis(fluorosulfonyl)imide and, more specifically, to a method for preparing lithium bis(fluorosulfonyl)imide, capable of simply and economically preparing lithium bis(fluorosulfonyl)imide, which is a lithium salt to be used in an electrolyte solution for a lithium secondary battery, in a high yield and with high purity. According to the present invention, the novel method for preparing lithium bis(fluorosulfonyl)imide can resolve the problem of a conventional technique by reacting a bis(chlorosulfonyl)imide compound, which is a starting material, with a fluorination reagent, and then immediately treating the same with an alkali reagent without purification or concentration, and has an effect of enabling lithium bis(fluorosulfonyl)imide to be simply and economically prepared in a high yield and with high purity.