The rapid development of modern portable electronics and electric vehicles toward safety and high reliability puts forward stringent requirements for fast ion conduction, high thermal/mechanical reliability, and easy processing and mass production of electrical insulating separators. Herein, inspired by the Janus separator (JS) with asymmetric properties on each side, we report a large-area asymmetrically structured p-aramid nanofiber (ANF)-based JS (ANF-JS) containing a dense nanoporous layer and a microporous layer, which is fabricated by a facile dry-wet phase inversion approach. The dense nanoporous layer imparts ANF-JS with good ion-selective, exceptionally high ionic conductivity, and insignificant lithium polysulfide (PS) shuttling. The microporous layer with interconnected and three-dimensionally porous structures provides additional channels for significant electrolyte uptake and fast Li-ion transport. Meanwhile, the excellent electrochemical properties are integrated with lightweight (0.7 mg cm⁻²), good electrochemical stability, nonflammability, a high modulus (up to 690 MPa), and excellent thermal dimensional stability (thermal shrinkage of ∼0% at 200 °C), much better than various conventional separators. Additionally, we successfully demonstrate its continuous and mass production in the form of a thin separator (8–20 μm), achieving outstanding cycle stability with a capacity decay as low as 0.0195% per cycle over 1000 cycles in Li–S batteries.