Due　to　the　high　ionic　conductivity　and　low　cost,　Li1.3Al0.3Ti1.7(PO4)(3)　(LATP)　has　emerged　as　a　promising　solid-state　electrolyte　for　next-generation　Li　metal　solid-state　batteries.　However,　the　susceptibility　of　LATP　to　side　reactions　with　Li　metal　and　the　significant　resistance　at　grain　boundaries　pose　major　challenges　for　the　applications　of　all-solid-state　batteries.　In　this　work,　the　non-equilibrium　microstructure　at　the　interface　of　a　LATP　composite　solid　electrolyte　(84　wt%　LATP　and　16　wt%　LiTFSI)　was　adjusted　by　two-step　cold　sintering,　reducing　interfacial　transport　barriers　for　optimizing　ionic　transport　paths　and　reducing　sensitivity　to　Li　metal　anodes,　thus　increasing　the　ionic　conductivity　and　interfacial　stability　of　solid　electrolytes.　While　preserving　the　original　grain　structure,　the　grain　boundary　resistance　was　significantly　reduced,　resulting　in　an　impressive　ionic　conductivity　of　7.3　x　10(-4)　S　cm(-1).　Additionally,　the　solid　electrolyte　had　a　high　density　of　97%　and　demonstrated　excellent　interfacial　compatibility　and　electrochemical　stability　by　maintaining　stable　cycling　for　2300　h　at　0.1　mA　h　cm(-2).　This　further　demonstrates　the　favorable　effect　of　the　non-equilibrium　microstructure　on　ion　transport　across　grain　boundaries　and　enhancing　interfacial　stability,　which　is　crucial　for　achieving　high　performance.　In　conclusion,　two-step　cold　sintering,　with　its　unique　process,　offers　significant　advantages　in　modulating　the　interfacial　microstructure　and　presents　a　novel　solution　for　interfacial　engineering.