Harvesting electric power from the salinity gradient has drawn the eyes of researchers in recent years, because it is sustainable and environmentally benign. Nanofluidic channels are regarded as a promising platform to utilize this clean energy, due to their unique fluidic transport properties in the nanometer scale. Therefore, technological breakthroughs are expected in exploitation of new types of nanofluidic channel membranes. Polymer/MOF hybrid membranes combine the advantages of abundant pore channels from MOF and high density of functional groups from polymers, which make them competitive membrane materials for the control of nanofluidic transport. Herein, we developed a series of hybrid nanochannel membranes constructed by polystyrene sulfonate (PSS)/MOF composites and anodic aluminum oxide (AAO). The resultant membranes feature geometrical, chemical, and electrostatic asymmetries. Through adjusting the PSS content in the hybrid nanochannel membranes, the optimized membrane exhibits outstanding cation selectivity and can rectify the ion current with a ratio of 98 in 10 mM KCl solution. After integrating it into a salinity-gradient-driven device, a high power density of 2.87 W/m2 is achieved, which shows great promise for practical applications. This work paves the way for the use of polymer/MOF composites in nanofluidic systems and boosts their applications in energy conversion areas.