Understanding the electrocatalyst–electrolyte interface, including electric double layer (EDL) effects, is critical for CO2 electroreduction (CO2RR). However, modeling the EDL’s full complexity, spanning large spatiotemporal scales (∼10 nm, >100 ps) remains a challenge for conventional simulations. Here, we integrate the grand canonical density functional theory (GC-DFT) with large-scale classical molecular dynamics and free energy perturbation (FEP) methods (30,000+ atoms, ns time scale) to model CO2 reduction to CO on a Ni–N–C/G catalyst under an applied potential of −0.60 VRHE in a 0.5 M KHCO3 electrolyte. The simulations indicate that under these specific conditions, the EDL substantially promotes CO2 adsorption (−0.64 eV) and facilitates the two proton-transfer steps while slightly inhibiting CO desorption. Moreover, the FEP simulation results reveal that the interfacial electric field (EF), rather than cation coordination, is primarily responsible for modulating these reaction energetics. Furthermore, a linear correlation is found between the perpendicular dipole moment change (Δμz) of adsorbed intermediates and the EF-induced free energy shift (ΔGFEP), suggesting a useful descriptor for assessing EDL influences. This work demonstrates the value of a multiscale framework for probing interfacial electrochemical phenomena.