Although promising strategies have been developed to resolve the critical drawbacks of lithium-sulfur (Li-S) batteries, the intractable issues including undesirable shuttling of polysulfides and sluggish redox reaction kinetics have still been unresolved thoroughly. Herein, a cobalt single-atom (CoSA) catalyst comprising of atomic Co distributed homogeneously within nitrogen (N)-doped porous carbon (Co-NPC) nanosphere is constructed and utilized as a separator coating in Li-S batteries. The Co-NPC exposes abundant active sites participating in sulfur redox reactions, and remarkable catalytic activity boosting the rapid polysulfide conversions. As a result, Li-S batteries with Co-NPC coating layer realize significantly enhanced specific capacity (1295 mAh·g⁻¹ at 0.2 C), rate capability (753 mAh·g⁻¹ at 3.0 C), and long-life cyclic stability (601 mAh·g⁻¹ after 500 cycles at 1.0 C). Increasing the areal sulfur loading to 6.2 mg·cm⁻², an extremely high areal capacity of 7.92 mAh·cm⁻² is achieved. Further in situ X-ray diffraction, density functional theory calculations, and secondary ion mass spectrometry confirm the high catalytic capability of CoSA towards reversible polysulfide conversion. This study supplies new insights for adopting single-atom catalyst to upgrade the electrochemical performance of Li-S batteries.