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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/HNO _3 , 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.