Heteroatom doping has emerged as a prevailing strategy to enhance the storage of sodium ions in carbon materials. However, the underlying mechanism governing the performance enhancement remains undisclosed. Herein, we fabricated N/S co-doped carbon beaded fibers (S-N-CBFs), which exhibited glorious rate performance and durableness in Na⁺ storage, showcasing no obvious capacity decay even after 3500 cycles. Furthermore, when used as anodes in sodium-ion capacitors, the S-N-CBFs delivered exceptional results, boasting a high energy density of 225 Wh·kg^–1, superior power output of 22500 W·kg^–1, and outstanding cycling stability with a capacity attenuation of merely 0.014% per cycle after 4000 cycles at 2 A·g^–1. Mechanistic investigations revealed that the incorporation of both pyridinic N and pyrrolic N into the carbon matrix of S-N-CBFs induced internal electric fields (IEFs), with the former IEF being stronger than the latter, in conjunction with the doped S atom. Density functional theory calculations further unveiled that the intensity of the IEF directly influenced the adsorption of Na⁺, thereby resulting in the exceptional performances of S-N-CBFs as sodium-ion storage materials. This work uncovers the pivotal role of IEF in regulating the electronic structure of carbon materials and enhancing their Na⁺ storage capabilities, providing valuable insights for the development of more advanced electrode materials.