Lithium-sulfur (Li-S) batteries are very promising high-energy-density electrochemical energy storage devices, but suffer from serious Li polysulfde (LiPS) shuttle and uncontrollable Li dendrite growth. Here, we show in situ polyolefn separator modifcation with an N-rich conjugated microporous polymer (NCMP) for advanced Li-S battery. In situ polymerization generates an ultrathin NCMP coating on the whole external surface and the internal surface of the separator, which is substantially different from the conventional approaches with thick coatings only on the external surface. The NCMP coating with abundant N-containing groups (-NH2 and -N=), uniform nanopores (12.294 Å), and π-conjugated structure can simultaneously inhibit LiPS shuttle and regulate uniform nucleation and growth of Li dendrites. Consequently, the NCMP-based separator endows the Li-S battery with signifcantly enhanced cycling stability at high S loading (5.4 mg cm-2), lean electrolyte (E/S = 6.3 μL mg-1), and limited Li excess (50 μm).