A preparation method of a high-voltage-resistant solid polymer electrolyte includes the steps of: 1) according to certain ratio, dissolving a polymer substrate, lithium salt, and an inorganic additivein anhydrous acetonitrile and stirring the mixture at room temperature to obtain a uniform solution, wherein the polymer substrate is polyoxyethylene, the lithium salt is lithium bistrifluoromethylsulfonimide or lithium perchlorate; the inorganic additive is nano-wires or nano-particles of zinc borate, aluminum borate, sodium tetraborate, barium metaborate or calcium borate, the mass ratio of thepolyoxyethylene to the lithium salt EO : Li<+> is 10-20:1, and the mass of the inorganic additive is not more than 20% of the total mass of the polymer substrate and lithium salt; 2) pouring the solution into a polytetrafluoroethylene mold to volatilize the solution until completely dried, thus preparing the solid polymer electrolyte. The method improves the high-voltage resistance of the solid polymer electrolyte, so that the product fits a high-voltage ternary cathode material and energy density and safety of an all-solid-state battery are improved.