The development of efficient, stable, and cost-effective metal-free photocatalysts is highly urgent for practical CO2 photoreduction, yet conventional photocatalysts typically suffer from insufficient active sites, poor mass transfer, and rapid charge recombination. Herein, self-nitrogen-functionalized hierarchical porous carbon spheres (PACs) were rationally synthesized via one-step suspension polymerization followed by mild potassium salt-assisted activation, using acrylonitrile as the sole carbon and nitrogen precursor. The pore structure engineering strategy effectively optimized the spatial distribution and accessibility of nitrogen-doped sites, enabling the synergistic integration of hierarchical porosity, large specific surface area, and high nitrogen content within a single carbon framework. As a result, the PACs prepared at 700°C with potassium acetate activation exhibited outstanding photocatalytic CO2-to-CO conversion activity and exceptional cycling durability. The CO yield reached 18.74 μmol g-1, remarkably higher than the pristine counterpart (10.63 μmol g-1), and 96.74% of the initial activity was preserved even after 20 consecutive cycles. Systematic characterizations revealed that the hierarchical porous architecture not only exposed abundant accessible nitrogen active sites but also strengthened CO2 adsorption and diffusion kinetics. Meanwhile, surface nitrogen species promoted interfacial charge separation and suppressed photogenerated electron-hole recombination. The synergistic effect between engineered porous structure and tunable nitrogen doping verified the crucial role of bidirectional “pore-nitrogen” regulation in boosting photocatalytic performance. This work provides a facile and mild activation route for constructing high-performance nitrogen-doped carbon-based metal-free photocatalysts, offering a promising paradigm for efficient and stable CO2 photoreduction.