Osmotic energy is considered as a promising large-scale, sustainable, and clean energy source. However, developing high-performance ion-selective membranes integrated with high power density, robust mechanical properties, and outstanding stability for osmotic energy harvesting remains a great challenge. Herein, we present a Janus membrane composed of common aramid nanofibers (ANFs) and sulfonated aramid nanofibers (SANFs). The SANF was prepared though a ring-opening addition reaction of 1,4-butanesultone for the first time. When utilized for osmotic energy harvesting, owing to the synergistic effect of enhanced surface charge density and ionic diode effect, this Janus membrane demonstrates simultaneously a high power density of ~12.3 W m−2, excellent mechanical strength (~126 MPa), and superior stability. This work provides a versatile platform for designing advanced ion-selective membranes with tailored charge and structural properties for next-generation sustainable energy technologies.