The electrochemical reduction of CO2 to value-added chemicals renewable electricity is a promising and ecofriendly strategy to achieve the national strategic goal of “carbon peak and carbon neutrality” and solve the greenhouse effect. Due to the variety of products in CO2 electroreduction (CO2ER), catalytic selectivity has become a key factor in the design of electrode structure. Herein, a systematic investigation of CO2ER on the nanoporous gold films with different thicknesses prepared by the self-deposition method developed by ourselves. Mass transfer effects are found to play an important role in determining product selectivity and activity. The specific activity for CO evolution (jCO) with exponential declination has more dramatic tendency than the specific activity for hydrogen evolution (jH2) with linear decay with increasing nanoporous gold film thickness. Different from the behaviors within the mesoporous structures in previous studies, the retarded transport of HCO3− ions within the nanoscale pores is more sensitive than that of protons. This phenomenon implies the necessity of considering mass transfer effects in the design of outstanding electrocatalysts for CO2ER as well as for understanding the geometrical infrastructure-performance relationships.