The invention relates to a preparation method of a spiro-quaternary ammonium salt electrolyte. The preparation method is characterized by subjecting a cyclic amine, diol and a strong acid to condensation reaction at 120-180 DEG C for 8-16 hours in a polar organic solvent in the presence of a catalyst, and then purifying the reactant, thus obtaining the spiro-quaternary ammonium salt electrolyte, wherein the cyclic amine is any one of pyrrolidone, piperidine and morpholine; diol is 1,4-butanediol or 1,5-pentanediol; the strong acid is any one of tetrafluoroboric acid, trifluoromethanesulfonic acid and bis(trifluoromethanesulfonyl)imidic acid; the feed mole ratio of the cyclic amine to diol to the strong acid is 1 to (0.7-0.9) to (0.8-1). The preparation method has the advantages that the raw materials are cheap; the whole synthetic reaction is good in selectivity; the product has high yield and high purity; no halide ions are introduced in the reaction process, and the content of halogen in the obtained target product is less than 1ppm, so that the preparation method can meet the requirements of electrochemical energy storage electrolytes and is suitable for industrial production.