Nanoporous catalysts have demonstrated excellent potential in electrochemical degradation of NO₃⁻. This is attributed not only to their intrinsic electronic structure but also to the confinement effects introduced by their unique nanoscale structure. To explore and validate the impact of these confinement effects, this study investigates the confined microenvironment between the Cu nanopores electrode and NO₃⁻ solution by combining molecular dynamics simulations and first-principles calculations. A layered arrangement of H₂O and NO₃⁻ with a NO₃⁻-rich liquid layer near the walls was revealed, and the RDF result further dividing the NO₃⁻ layer into a reaction layer and a supply layer. The optimized interfacial microenvironment caused by confinement effects regulates the spatial positioning of NO₃⁻ and minimizes the free energy barriers for reduction, and proper NO₃⁻ concentration and a near-neutral environment favor the optimized catalytic microenvironment. Kinetic migration analysis reveals that the layered solution does not restrict particle transport. NO₃⁻ show relatively small migration coefficient due to high adsorption energy, while H₂O exhibits fast mass transfer, securing a consistent proton supply in reduction. This research provides valuable insights into nanochannel confinement effects and offers valuable insights for designing advanced catalytic systems and interfacial microenvironment with improved reaction rates and selectivity.