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The shuttle effect of polysulfides poses a significant obstacle to the commercialization of lithium-sulfur (Li–S) batteries. Covalent organic frameworks have been widely used in Li–S batteries as porous crystalline materials to suppress the shuttle effect. Herein, a 3,5-bis(trifluoromethyl)benzyl (BTFMB-TzDa) modified triazine-based covalent organic framework was synthesized. The high electronegativity and large steric hindrance of the BTFMB-TzDa modified separator successfully suppressed the diffusion of polysulfides, leading to improved capacity and cyclic stability of Li–S batteries. Cells with the BTFMB-TzDa modified separator exhibited a high initial capacity of 1205 mA h g−1 at 0.2C and 657 mA h g−1 at 3.0C. Furthermore, the capacity still remained at 501 mA h g−1 after 500 cycles. The BTFMB-TzDa modified separator provides an alternative strategy for the construction of high-performance Li–S batteries. [ABSTRACT FROM AUTHOR]