Interface　instability　between　the　perovskite　Li3xLa2/3-xTiO3　(LLTO)　and　lithium　metal　anode　limits　the　application　in　all-solid-state　lithium　batteries.　Direct　contact　with　lithium　metal　triggers　Ti4+　reduction　in　crystalline　LLTO,　including　rapid　lattice　destabilization　and　performance　decay,　though　the　governing　mechanisms　remain　unelucidated.　Herein,　significant　insights　from　multi-scale　characterization　technologies　and　molecular　dynamics　simulations,　reveal　the　formation　of　oxygen　vacancies　in　LLTO　and　lithium　oxides　at　the　interface.　The　mechanical　measurements　with　nanoindentation　and　Vickers　hardness　allow　us　to　directly　quantify　the　degradation　of　hardness　and　elastic　modulus　of　LLTO.　Li|LLTO|Li　symmetric　cells　further　confirm　the　electrochemical　decline.　We　propose　a　degradation　mechanism　of　the　interfacial　instability　from　the　perspective　of　microstructure,　mechanical,　chemical　and　electrochemical　kinetics.　This　study　provides　crucial　insights　into　the　microscopic　origin　to　understand　the　interfacial　chemical　processes,　which　is　helpful　to　guide　future　interface　designs.