The development of high-performance lithium–sulfur (Li–S) batteries is hindered by the complex interplay of lithium polysulfides (LiPSs) shuttle effects and uncontrolled lithium dendrite growth. Herein, we introduce a dual-functional—phosphorus-doped iron single-atom catalysts on reduced graphene oxide (Fe-NPC@rGO)—to address both issues. Density functional theory (DFT) and experiments reveal that Fe-NPC@rGO enhances sulfur redox kinetics and regulates lithium deposition. The Fe–NPC high charge density and enhanced electron transfer (vs. Fe–N4) enable Fe–NPC@rGO to trap polysulfides (LiPSs) and boost their conversion, reducing shuttle effects. Simultaneously, its lithiophilic properties enable uniform Li plating, inhibiting dendrites. Li–S cells with Fe–NPC@GO modified separators deliver a high discharge capacity of 1156 mAh g−1 at 1 C, with an exceptionally low-capacity decay of 0.032% per cycle over 1000 cycles. Moreover, full Li–S battery configuration (Fe−NPC@rGO-Li||Fe−NPC@rGO-PP||ROCNT-S) achieves high areal capacity of 4.9 mAh cm−2 at 5 mg cm−2 sulfur loading, low electrolyte to sulfur (E/S) ratio of 6 µL mg−1, and an ultralow negative to positive (N/P) ratio of 1.2. These findings provide valuable insights into the structural optimization of electrocatalysts and underscore the significant potential of Fe–NPC@rGO in advancing the electrochemical performance of next-generation Li–S batteries.