Lithium-sulfur (Li-S) batteries stand as an important candidate for next-generation high-energy secondary batteries due to its high specific capacity, low cost and environmental friendliness. However, practical application of Li-S batteries suffers from low rechargeability, poor rate capability and cycling instability of sulfur cathode, which can be mainly ascribed to the poor conductivity of sulfur and the dissolution of the intermediate polysulfides generated during discharge-charge cycles. In this work, a Nafion/super P-modified dual functional separator is designed to improve the long-term cycle stability and rate capability of the pure sulfur cathode. The electrostatic repulsion between the SO3- groups and the dissolved negative Sn2- ions, and the trap and reutilizing effect of super P for polysulfides, provide double insurance to confine the polysulfides within the cathode side, leading to great improvement in both reversible capacity and cycling stability of the sulfur cathode as compared to the battery with pristine Celgard separator. With such dual functional separator, a simple elemental sulfur cathode with 70% S content delivers a high initial discharge capacity of 1087 mAh g-1 at 0.1C and a long-term cyclability with only 0.22% capacity fade per cycle over 250 cycles at 0.5C.