Catalytic conversion of lithium polysulfides (LiPSs) is regarded as an effective approach to boosting kinetics and suppressing shuttling effect in lithium-sulfur (Li-S) batteries, but developing efficient catalysts and understanding of its catalytic mechanism still remain challenges. Herein, the ultrafine V8C7 nanocrystals embedded into the porous N-doped carbon frameworks (V8C7/NC) was designed for accelerating the LiPSs catalytic-conversion kinetics toward high-performance Li-S batteries. We propose that the ultrafine V8C7 nanocrystals with abundant unsaturated coordination atoms increase the active sites for catalytic-conversion of LiPSs and also accelerate the corresponding kinetics by reducing its reaction activation energy, meanwhile the interlinked nano-channels of V8C7/NC can effectively confine LiPSs and also promote electron/ion transfer. These attributes enable the Li-S cell equipped with V8C7/NC to deliver a large discharge capacity of 1393.3 mAh g−1 at 0.2 C, excellent rate capability (496.0 mAh g−1 at 10 C), and robust cycling stability (1000 cycles with a negligible capacity decay rate of 0.019 % per cycle even at 5 C). The innovative concept of ultrafine V8C7 nanocrystals embedded into the porous carbon toward improved LiPSs catalytic-conversion kinetics could be extended to develop other electrocatalytic hosts for high-performance Li-S batteries.