A solid polymer electrolyte and a method of manufacturing the same are provided. More particularly, a solid polymer electrolyte having a high content of solids and exhibiting a flame retardant property and a method of manufacturing the same, wherein the solid polymer electrolyte includes a multifunctional acrylate-based polymer, a C2 to C10 polyalkylene oxide, a lithium salt and a non-aqueous solvent and wherein the multifunctional acrylate-based polymer is cross-linked with the polyalkylene oxide to form a semi-interpenetrating polymer network (semi-IPN).