In the electrochemical CO₂ reduction reaction (CO₂RR), halide ions could impose a significant effect on multi-carbon (C₂₊) product production for Cu-based catalysts by a combined contribution from various mechanisms. However, the nature of specific adsorption of halide ions remains elusive due to the difficulty in decoupling different effects. This paper describes a facile method to actively immobilize the morphology of Cu-based catalysts during the CO₂RR, which makes it possible to reveal the fundamental mechanism of specific adsorption of halide ions. A stable morphology is obtained by pre-reduction in aqueous KX (X = Cl, Br, I) electrolytes followed by conducting the CO₂RR using non-buffered and non-specifically adsorbed K₂SO₄ as the supporting electrolyte, by which the change of local pH and cation concentration is also maintained during the CO₂RR. In situ spectroscopy revealed that the specific adsorption of halide ions enhances the adsorption of *CO intermediates, which enables a high selectivity of 84.5% for C₂₊ products in 1.0 M KI.