The invention discloses a lithium battery fire-extinguishing filling material additive preparation method. Preparation of a PEO-based fire-extinguishing additive comprises the steps that according toa mass ratio of 5 to 1, PEG and a silane coupling agent are accurately weighed, an appropriate amount of THF is used as a solvent for dissolving the PEG and the silane coupling agent, and after the PEG and the silane coupling agent are mixed, reaction is carried out under protection of nitrogen; a product, subjected to filtering treatment, of the last step is reacted with nano-silica hydrosol under protection of nitrogen, and a reaction product is subjected to centrifugation treatment; finally, the product of the last step is reacted with PPG. A product is observed through an electron microscope, a result shows that the product is good in reaction evenness, and both the dispersity and the mobility are up to standards; the novel high-flame retardation fire-extinguishing additive is easy tosynthesize, easy to use and low in waste, and a better choice is provided for replacing a toxic halogen-flame retardant.