The rapid development of electric vehicle industry has placed urgent need for solid state battery with large capacity and high energy density. The electrolyte and interface problems are two key issues that limit the performance of solid state battery. The paper introduced PEO buffer layer to the cathode/electrolyte interface to improve contact. First, Li6.4La3Zr1.4Ta0.6O12(LLZTO) with density of 5.25 g/cm3 and ionic conductivity of 8.33×10-4 S/cm was prepared. Second, the paper prepared PEO-LiTFSI-LLZTO buffer layer and LiFePO4 composite cathode slurry. The PEO buffer layer and cathode slurry were coated on the surface of electrolyte by spin coating, respectively. The interfacial resistance was 509 Ωcm2 at 60 °C. The galvanostatic charge and discharge curve of symmetric battery proved excellent interfacial stability. The solid state battery has fist discharge capacity of 145.8mAh/g and coulombic efficiency of over 97%.