Abstract: A lithium salt easily soluble in an organic solvent and having a flame-retardant function and a lithium-ion battery flame-retardant electrolyte thereof are provided. The lithium salt is poly(lithium phosphate) phosphazene partially substituted by alkyl aromatic oxy groups, and has a structural general formula: [(R—Ar—O)x(P?N)n(Li2O3P)2n-x]. The novel flame-retardant electrolyte is compounded from the lithium salt and a phosphate intermediate thereof [(R—Ar—O)x(P?N)n(R?2O3P)2n-x] according to a mass ratio of 10:1-1:1. The electrolyte is easily soluble in an organic solvent. A liquid electrolyte is prepared according to an amount of 8%-45% to obtain the novel flame-retardant liquid electrolyte. The liquid electrolyte has good lithium ion conductivity and good flame-retardant properties, and is used in lithium-ion batteries, lithium-sulfur batteries, lithium carbon fluoride batteries or lithium-oxygen batteries.