A solid-state electrolyte (SSE) with a sandwich structure, comprising a lithium battery separator and a first composite SSE and a second composite SSE that are coated on two side surfaces of the lithium battery separator, respectively; wherein the SSE with the sandwich structure is prepared by a method comprising mixing polyethylene oxide, a ceramic powder, an ionic liquid, a lithium salt, and a photoinitiator with an organic solvent, coating an obtained slurry on the two side surfaces of the lithium battery separator, and drying to obtain a film with a sandwich structure; and subjecting the film with the sandwich structure to hot pressing and ultraviolet crosslinking sequentially to obtain the SSE with the sandwich structure; wherein the polyethylene oxide has a weight-average molecular weight of 0.1 million to 7 million; the ceramic powder is one or more selected from the group consisting of lithium aluminum titanium phosphate, lithium lanthanum zirconium oxide, lithium lanthanum titanium oxide, lithium aluminum oxide, doped lithium lanthanum zirconium oxide, lithium germanium phosphorus sulfide, and lithium phosphorus sulfide; the lithium salt is one or more selected from the group consisting of lithium perchlorate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(trifluoromethanesulphonyl)imide, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, and lithium bis(oxalate)borate; the photoinitiator comprises a hydrogen abstraction photoinitiator; the ionic liquid is an imidazole-based ionic liquid or a pyrrolidine-based ionic liquid, the imidazole ionic liquid being 1-ethyl-3-methylimidazole bis(trifluoromethanesulphonyl)imide or 1-ethyl-3-methylimidazole tetraborate, and the pyrrolidine-based ionic liquid being 1-n-butyl-1-methylpyrrolidine bis(trifluoromethanesulphonyl)imide; and the PEO, the ceramic powder, the ionic liquid, the lithium salt, and the photoinitiator are at a mass ratio of (20-90):(5-15):(10-60):(10-50):(0.5-2). mixing polyethylene oxide, a ceramic powder, an ionic liquid, a lithium salt, and a photoinitiator with an organic solvent, coating an obtained slurry on the two side surfaces of the lithium battery separator, and drying to obtain a film with a sandwich structure; and subjecting the film with the sandwich structure to hot pressing and ultraviolet crosslinking sequentially to obtain the SSE with the sandwich structure; wherein the polyethylene oxide has a weight-average molecular weight of 0.1 million to 7 million; the ceramic powder is one or more selected from the group consisting of lithium aluminum titanium phosphate, lithium lanthanum zirconium oxide, lithium lanthanum titanium oxide, lithium aluminum oxide, doped lithium lanthanum zirconium oxide, lithium germanium phosphorus sulfide, and lithium phosphorus sulfide; the lithium salt is one or more selected from the group consisting of lithium perchlorate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(trifluoromethanesulphonyl)imide, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, and lithium bis(oxalate)borate; the photoinitiator comprises a hydrogen abstraction photoinitiator; the ionic liquid is an imidazole-based ionic liquid or a pyrrolidine-based ionic liquid, the imidazole ionic liquid being 1-ethyl-3-methylimidazole bis(trifluoromethanesulphonyl)imide or 1-ethyl-3-methylimidazole tetraborate, and the pyrrolidine-based ionic liquid being 1-n-butyl-1-methylpyrrolidine bis(trifluoromethanesulphonyl)imide; and the PEO, the ceramic powder, the ionic liquid, the lithium salt, and the photoinitiator are at a mass ratio of (20-90):(5-15):(10-60):(10-50):(0.5-2). <EOS>