The development of membrane science plays a fundamental role in harvesting osmotic power, which is considered a future clean and renewable energy source. However, the existing designs of the membrane cannot handle the low conversion efficiency and power density. Theory has predicted that the Janus membrane with ionic diode–type current would be the most efficient material. Therefore, rectified ionic transportation in a hypersaline environment (the salt concentration is at least 0.5 M in sea) is highly desired, but it still remains a challenge. Here, we demonstrate a versatile strategy for creating a scale-up Janus three-dimensional (3D) porous membrane–based osmotic power generator system. Janus membranes with tunable surface charge density and porosity were obtained by compounding two kinds of ionomers. Under electric fields or chemical gradients, the Janus membrane has ionic current rectification properties and anion selectivities in a hypersaline environment. Experiments and theoretical calculation demonstrate that abundant surface charge and narrow pore size distribution benefit this unique ionic transport behavior in high salt solution. Thus, the output power density of this membrane-based generator reaches 2.66 W/m² (mixing seawater and river water) and up to 5.10 W/m² at a 500-fold salinity gradient (i.e., flowing salt lake into river water). Furthermore, a generator, built by connecting a series of membranes, could power a calculator for 120 hours without obvious current decline, proving the excellent physical and chemical stabilities. Therefore, we believe that this work advances the fundamental understanding of fluid transport and materials design as a paradigm for a high-performance energy conversion generator.