The invention belongs to the technical field of solid electrolytes, and particularly relates to a composite solid electrolyte and a preparation method thereof. The method comprises the steps: enablingpolyether glycol, 2, 6-toluene diisocyanate, a catalyst and a chain extender to be mixed and react to obtain a polyurethane prepolymer; in an inert gas atmosphere, dispersing a lithium salt and an inorganic solid electrolyte into 1, 3-dioxolane, adding an initiator and a plasticizer, and performing uniform dispersing to obtain a precursor solution; and adding the precursor solution into the polyurethane prepolymer, performing stirring and mixing uniformly, performing standing, performing heating to 70-75 DEG C, and keeping the temperature for 6-8 hours to obtain the composite solid electrolyte. According to the invention, the 1, 3-dioxolane takes the lithium salt additive as a ring-opening initiator, the coating of the inorganic solid electrolyte is performed in the ring-opening polymerization process, and a mutually doped networked polymer with a polyurethane system is formed, so the formed networked porous structure has a rich ion transport network, and the ion transport property and the electron conductivity are improved.