The activity and selectivity of the carbon dioxide reduction reaction (CO₂RR) can be significantly enhanced by altering the electronic structure of central transition-metal atoms in single-atom catalysts (SACs). However, the role of mass transport in catalyst design, though critical, is frequently overlooked. Herein, a single-atom Ni-N-C(P)-8 catalyst featuring hierarchical micropores and mesopores is synthesized to investigate the role of the mass transport process in CO₂RR. Remarkably, the mesopores-rich catalyst can efficiently decrease the mass transport barrier, achieving a high CO Faradaic efficiency (FE) of 99 % at −0.7 V vs. reversible hydrogen electrode (RHE) and turnover frequencies (TOFs) for CO production of 31644 h⁻¹ at −0.9 V vs. RHE. The detailed experiments and classical molecular dynamics simulations reveal that the abundance of mesoporous pores on the catalyst surface significantly enhances the mass transport process to the active Ni sites during the reaction, and thereby enhancing the CO production rate.