The flammable organic liquid electrolytes in Li-ion batteries may incur serious safety issues, all-solid-state batteries are regarded as the ultimate solution to the safety issues. Among various types of solid electrolytes, solid polymer electrolytes have been widely studied because of their advantages, such as good filming property, cost-effective, and mechanical flexibility. However, the low Li-ion conductivity and instability of solid polymer electrolytes hinder their actual applications in the high energy density batteries. An effective strategy to address these issues is to composite ceramic electrolytes and polymer electrolytes. Here, a three-dimensional framework Li_6.28La₃Zr₂Al_0.24O₁₂ (3D-LLZAO) inorganic electrolyte was prepared by sacrificial template method, and was used to construct a polyethylene oxide (PEO) based composite solid state electrolyte film. The 3D-LLZAO was fabricated by using a polymeric sponge method, namely impregnating the polymeric sponge with an aqueous ceramic slurry. PEO:LiTFSI (lithium bis((trifluoromethyl)sulfonyl)azanide) solution was obtained by dissolving PEO (M_w=1×10⁶) and LiTFSI in acetonitrile with a molar ratio of n(EO)∶n(Li⁺)=18∶1. After that, succinonitrile (SN) is added with a mass ratio of m(PEO)∶m(SN)=2∶1. The 3D garnet composite electrolyte was acquired by immersing the 3D garnet framework of different qualities into the PEO-LiTFSI-SN solution in vacuum. The prepared solution drops are added to the teflon template. Afterward, the sample was dried in vacuum to remove the solvent. The properties of PEO based composite solid state electrolyte were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV) and charge-discharge cycle. The results show that the PEO based composite solid state electrolyte CPE-10 with 10% (w) 3D-LLZAO has smaller volume resistance and wider electrochemical stability window. The ionic conductivity of the composite electrolyte at room temperature is 1.58×10⁻⁴ S•cm⁻¹, and the lithium-ion migration number is 0.26. The symmetric battery assembled with a composite solid electrolyte can be stably circulated for 1600 h at a current density of 0.05 mA•cm⁻² at room temperature. The assembled LiFePO₄/CPE-10/Li battery shows an initial specific discharge capacity of 155.6 mAh•g⁻¹ at a rate of 0.5 C, and a capacity retention rate of 86% after 100 cycles. It turns out that 3D garnet composite electrolytes are good candidates for next-generation lithium metal batteries.