Lithium-sulfur (Li-S) battery has been regarded as one of the most promising next-generation energy storage systems, due to its high energy density and low cost. However, the dissolution of polysulfides to electrolyte and consequent “shuttle effect” still hinder the application of Li-S battery. Here, a polyethylenimine/fish-scale based porous carbon composite (PFPC) with abundant active sites and a relatively compact morphology is prepared by wet impregnation method. Modified separator with the PFPC coating significantly improves the rate capability and cycle stability of Li-S battery. The Li-S battery with the PFPC separator shows a reversible capacity of 638 mAh g−1 after stably cycling for 400 cycles at 1.0 C (1.0 C = 1675 mA g−1) with an ultralow capacity fading rate of 0.030% per cycle. The improved electrochemical performance of Li-S battery can be attributed to the synergistic effect of physical blocking and chemical adsorption of PFPC for polysulfides and the fast electron conduction between adsorbed polysulfides and conductive network. Moreover, the PFPC separator is ultralight with a mass loading of about 0.04 mg cm−2. The good electrochemical performance, facile preparation method and light weight make the PFPC separator promising for the Li-S battery application.