Garnet-type　electrolytes　have　attracted　special　attention　due　to　their　excellent　stability　to　lithium　metal　and　high　ionic　conductivity.　However,　solid-solid　contact　and　the　interaction　of　lithium　ions　with　surface　electrons　of　the　electrolyte　produces　uncontrolled　lithium　dendrites,　resulting　in　poor　cycle　stability　and　safety　problems,　and　limiting　its　future　use　in　solid-state　batteries.　In　this　study,　we　reduce　the　interfacial　impedance　and　interfacial　charge　accumulation　by　constructing　an　in　situ　electrochemical　nanoscale　Li-Ag　alloying　interface,　which　solves　the　Li/garnet　interface　compactness　and　lithium　dendrite　concerns.　The　in　situ　nanoscale　Li-Ag　alloy　interface　is　formed　by　first　magnetron　sputtering　a　20-nm-thick　Ag　layer　on　the　solid　electrolyte,　and　then　alloyed　with　Li　by　galvanostatic　electrochemical　treatment,　which　effectively　improves　the　interfacial　compactness　between　lithium　metal　and　the　solid　electrolyte　and　the　uniform　deposition　of　lithium　ions,　as　well　as　significantly　reducing　interfacial　resistance　and　inhibiting　lithium　dendrite　growth.　The　above　interfacial　modification　decreases　Li/garnet　resistance　from　42614　S2　cm2　to　2　S2　cm2,　and　the　Li/LLZTO@Ag/Li　symmetric　cell　delivers　a　high　critical　current　density　of　5.1　mA　cm-　2.　The　solid-state　LiFePO4　(LFP)/LLZTO@Ag-20　nm/Li　battery　performs　a　specific　capacity　of　150　mAh　g-　1　at　0.1C　and　a　high　capacity　retention　of　90.6　%　after　300　cycles　at　0.5C.　The　nanoscale　alloy　interfacial　layer　assists　in　improving　comprehension　of　charge　accumulation　behavior　at　the　Li/garnet　interface　and　provides　more　inspiration　for　further　realization　of　solid-state　batteries　with　high　energy　and　safety.