Dendrite formation on lithium (Li) metal anode is a key issue which hinders the development of rechargeable Li battery seriously. A novel method for suppressing Li dendrites via using Li phosphorous oxynitride (LiPON) modified Li anode and Li_1.5Al_0.5Ge_1.5(PO₄)₃-poly(ethylene oxide)(Li bistrifluoromethane-sulfonimide) (LAGP-PEO(LiTFSI)) composite solid electrolyte in all-solid-state Li battery is proposed, and the effect of the thickness of LiPON on Li anode performance is also studied. LiPON film with a thickness of 500 nm exhibits satisfactory interface property between Li metal anode and the LAGP-PEO(LiTFSI) solid electrolyte. The LiPON film provides a uniform Li⁺ flux across the interface and effectively inhibits the formation of Li dendrites in all-solid-state Li batteries. The assembled all-solid-state Li cell Li(LiPON)/LAGP- PEO(LiTFSI)/LiFePO₄ delivers an initial discharge capacity of 152.4 mAh·g^-1 and exhibits good cycling stability and rate performance at 50 ℃.