Lithium　niobate　material　(LN)　has　shown　great　application　potentials　in　the　fabrication　of　integrated　optical　devices　due　to　its　excellent　physical　properties,　especially　with　the　occurrence　of　lithium　niobate-on-insulator　(LNOI)　substrate.　However,　the　greatest　challenge　of　micro/nano　optical　devices　based　on　LN　material　lies　in　the　precise　etching　process　and　thus　limits　its　applications.　In　this　paper,　we　comprehensively　analyze　the　etching　results　treated　by　the　proposed　proton-exchanged　wet-etching　method　(PEWE)　combining　with　theoretical　simulations　and　experiments.　It　is　found　that　the　proton-exchanged　layer　in　the　LN　material　can　be　easily　etched　after　using　a　mixture　acid　of　HF/HNO3,　leading　to　the　improvement　of　etching　rate　and　surface　morphology.　The　lowest　roughness　of　the　optical　waveguide　is　measured　to　be　0.81　nm,　which　is　beneficial　for　the　performance　improvement　of　LN-based　optical　devices.　Ultimately,　a　quasi-vertical　sidewall　of　the　upper　part　of　optical　waveguide　with　improved　surface　morphology　is　successfully　realized　by　utilizing　the　PEWE.　Moreover,　this　method　could　also　be　extended　to　improve　the　performance　of　LNOI-based　optical　devices　and　pave　the　way　for　ultra-compact　photonic　integrated　circuits　based　on　LNOI.