The effects of an Li-Al layer on the modified-Li anode based on solid state batteries are studied. The structure, surface morphology, and cyclic stability of the modified-Li anode are examined by X-ray diffraction spectroscopy, scanning electron microscopy, and electrochemical tests. Results show that interphase layer can prevent Li from reacting with Li1.5Al0.5Ge1.5(PO4)3 and displays a much better cycling stability compared with the bare-Li anode used in symmetric cells. The assembled solid state batteries with modified-Li anode and LiMn0.8Fe0.2PO4 cathode can deliver an initial discharge capacity of 153 mAh g−1 with good cyclic stability and rate performance at 50 °C. The reason for this is that the Li-Al alloy interphase layer can prevent localised Li enrichment, then suppress lithium dendrite growth, and the Li-Al layer on the anode possesses good stability in the presence of Li1.5Al0.5Ge1.5(PO4)3 powder and better interphase contact results between the anode and solid electrolyte in solid state batteries.