CO₂ electroreduction reaction (CO₂RR) has been considered as an effective technology to close the anthropogenic carbon cycle. Formate, a product of two-electron transfer in CO₂RR, is an economically valuable feedstock. In this work, nanoporous tin oxides were controllable synthesized by a facile and scalable electrochemical anodic oxidation method. XPS result indicated that the increased Sn⁴⁺ species after anodic oxidation were beneficial to reduce the overpotential of formate formation. Operando Raman spectra revealed that the enhanced formate selectivity could be attributed to the high local pH within the porous structure, which suppresses hydrogen evolution reaction (competing reaction against CO₂RR). Further flow cell test showed a formate partial current density of 285 mA cm⁻² with the selectivity of 96.4%, indicating a promising industrial application prospect.