The invention provides a flame-retardant gel electrolyte for a lithium-ion battery. The flame-retardant gel electrolyte is mainly prepared by thermal polymerization of an electrolyte lithium salt, an organic solvent and a functional additive; the flame-retardant gel electrolyte mainly is of a polymer network structure formed by an organic polymer; and the electrolyte lithium salt, the organic solvent and the functional additive are uniformly dispersed into swelling gel. The preparation method of the lithium-ion battery comprises the steps of firstly dissolving the electrolyte lithium salt into the organic solvent, dividing the solution into two parts, adding a flame retardant to one part to obtain a mixed solution A and adding a monomer initiator to the other part to obtain a mixed solution B; preparing a positive plate and a negative plate; preparing a battery core from the positive plate, the negative plate and a diaphragm in a winding manner, putting the battery core into an aluminum-plastic film, mixing the mixed solution A with the mixed solution B evenly and injecting the mixed solution into the battery core; and carrying out vacuum sealing and room-temperature standing on the battery core and carrying out thermal polymerization reaction to obtain the lithium-ion battery filled with the flame-retardant gel electrolyte. The safety performance of the lithium-ion battery is significantly improved.