CO2 electroreduction to high-energy-density C2+ products is highly attractive, whereas the C2+ selectivity under industrial current densities is still unsatisfying. Here, an anti-swelling anion exchange ionomer (AEI) was first proposed to optimize the local environment for promoting industrial-current-density CO2-to-C2+ electroreduction. Taking the anti-swelling AEI-modified oxide-derived Cu nanosheets as an example, in situ Raman spectroscopy and contact angle measurements revealed that the OH--accumulated -N(CH3)3+ groups and anti-swelling backbone of AEI could synergistically regulate the local pH level and water content. In situ Fourier-transform infrared spectroscopy and theoretical calculations demonstrated that the higher local pH value could lower the energy barrier for the rate-limiting COCO* hydrogenated to COCOH* from 0.08 to 0.04 eV, thereby boosting the generation of C2+ products. Owing to the anti-swelling backbone, the optimized water content of 3.5% could suppress the competing H2 evolution and hence facilitate the proton-electron transfer step for C2+ production. As a result, the anti-swelling AEI-modified oxide-derived Cu nanosheets achieved a C2+ Faradaic efficiency of 85.1% at a current density up to 800 mA cm-2 with a half-cell power conversion efficiency exceeding 50%, outperforming most reported powder catalysts.