Carbon nanospheres enriched with α-MoC_1-x nanocrystalline (α-MoC_1-x/CNS) were synthesized by self-assembly and applied as a mediator for the surface of commercial polypropylene (PP) separator. Compared with pristin PP separator, the cycling stability and rate performance of the lithium-sulfur batteries with the modified α-MoC_1-x/CNS-PP separator are significantly improved and the battery with α-MoC_1-x/CNS-PP separator exhibits an initial discharge capacity of 1129.7 mAh/g at 0.5C and retains 855.5 mAh/g after 100 cycles with above 98% Coulombic efficiency. Remarkably, the capacity loss rate is only 7.7% after 48 h static storage. Combined with the morphology and XPS analysis of α-MoC_1-x/CNS, it is found that the designed α-MoC_1-x/CNS-PP separator prevents the migration of lithium polysulfide to the anode during the process of charge and discharge in lithium sulfur batteries. Formations of Mo-S bonds, thiosulfate and polythionate are attributed to the contact between lithium polysulfide and α-MoC_1-x/CNS, which further restrains active material in cathode region and improves the performance of lithium- sulfur batteries consequently.