Lignin-based hard carbon has emerged as a promising anode material for sodium-ion batteries due to its abundant availability, cost-effectiveness, and favorable electrochemical properties. Among various structural optimizations, heteroatom-doping and the formation of closed pores in hard carbon are widely recognized as key strategies for enhancing electrochemical performance. However, a comprehensive understanding of the molecular level mechanisms of closed pores formation in doped hard carbon remains elusive. Herein, an effective strategy involving N-doping followed by free radical generation is employed to create interconnected closed pores in lignin-derived hard carbon, enhancing their structural and electrochemical properties. In the first step, melamine is grafted onto highly reactive lignin, embedding N atoms into the lignin carbon matrix during carbonization. This process generates more defects and active sites, with the incorporation of 3 types of N-doped structures. Pyridinic-N and quaternary-N contribute to increasing sp² and sp³ hybridization, and the electrostatic repulsion of pyrrolic-N expands the interlayer spacing. In the second step, melamine-modified lignin is grafted with polyacrylamide (PAM), exposing sufficient free radicals during co-pyrolysis. Excessive free radicals generated from the degradation create more reactive sites, competing for limited precursor debris. This competition leads to the formation of smaller microcrystals and facilitates the development of cyclic intermediates. This structural reorganization facilitates the development of closed pores during carbonization, which is a crucial factor for enhancing capacity. As a result, the optimized sample exhibits a large volume of closed pores, resulting in a high reversible capacity and enhanced sodium ion transfer kinetics. This study presents a novel strategy for regulating hard carbon nanostructures and proposes an in-depth investigation of the formation mechanisms of closed pores for N-doped hard carbon, offering valuable insights into the rational design of hard carbon pore structures.