Lithium-sulfur batteries (LSBs) have drawn intense attention due to their high theoretical specific energy and low material cost. However, shuttle effect of the lithium polysulfides (LiPSs) dissolved in the electrolyte results in severe capacity fading and self-discharge. Herein, a multifunctional modifier composed of mesoporous boron carbon nitride and conductive graphene (BCN/G) has been developed to modify separator as an efficient barrier for LiPSs in LSBs. Benefiting from such a modified separator, LSBs exhibit improved cycling capacity and high rate capacities. The excellent electrochemical performances are mainly attributed to the strong chemical interactions between LiPSs and conductive BCN/G modifier, which have been confirmed by X-ray photoelectron spectroscopy analyses. Such a design of separator opens a new approach for high-performance LSBs.