The invention provides a preparation method of composite polymer electrolyte. The preparation method comprises the following steps: (A) mixing a polymer matrix, an alkali metal salt and a solvent to obtain a mixed solution; mixing a sulfide electrolyte raw material with the solvent; heating and stirring; reacting to obtain sulfide electrolyte precursor slurry; (B) mixing the mixed solution with the sulfide electrolyte precursor slurry to obtain a composite electrolyte solution; and (C) after drying the composite electrolyte solution, carrying out heat treatment to obtain the composite polymer electrolyte. The composite polymer electrolyte is prepared by adopting a liquid-phase method and sulfide electrolyte is more finely and more uniformly distributed on the polymer matrix; and finally, the composite polymer electrolyte has relatively good chemical stability and ion conductivity.