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Sluggish redox kinetics, shuttle effect, poor conductivity, and large volume change of sulfur limit the practical applications of lithium‐sulfur batteries. Hollow, porous, and necklace‐like Fe3C/N‐codoped carbon nanoboxes (Fe3C/NC) connected by N‐doped carbon (NC) nanofibers are designed by pulverizing Fe2O3 embedded in polyacrylonitrile (PAN) fibers to produce multifunctional sulfur hosts, which exhibit multiple polysulfide anchoring and catalytic conversion activities. Experimental and first‐principles density functional theory studies reveal that the uniformly distributed Fe3C and N units in the nanoboxes can significantly suppress the polysulfide shuttle effect. The conversion of polysulfides (LiPSs) to Li2S is catalyzed during discharge. The process relies on the fast electron transfer through the NC nanofibers and facilitated Li+ diffusion through the porous nanobox shells. The structural characteristics (“boxes in fibers”) of the nanoboxes influence the high sulfur loading and tolerance of volume variation of LiPSs, resulting in the synergistic catalysis of the redox reactions. A high capacity of 645 mAh g−1 after 240 cycles at 1 C, a high capacity of 712 mAh g−1 at a high sulfur loading of 5 mg cm−2 after 100 cycles at 0.2 C, and an enhanced areal capacity of 3.6 mAh cm−2 are demonstrated.