This study develops a covalent–metal organic framework (MOF@COF) heterogeneous membrane for diffusion-osmotic energy conversion. Conventional osmotic energy conversion systems often suffer from low voltage and current output, limiting overall energy conversion efficiency. To address this issue, we propose a MOF@COF heterogeneous membrane that integrates diffusion-osmotic and ionic rectification effects to enhance ion transport and boost power generation performance. The MOF@COF membrane features tunable nanopore structures and surface functionalization, improving ion selectivity and directional transport capability. The COF thin film was fabricated via a drop-casting technique, followed by in-situ MOF growth to form the heterogeneous structure. Electrochemical measurements were conducted to evaluate ion selectivity and power generation performance under various salinity gradients and ionic conditions. Results show that the COF membrane prepared with a 1.5 mL casting solution exhibits excellent structural integrity and stability, achieving a high power density of approximately 7.4 W/m² under a 50-fold NaCl concentration gradient (compared to ~5 W/m² for commercial standards). After growing MOF onto the COF surface to form the MOF@COF structure, both ionic rectification and selectivity were further enhanced, yielding a maximum power density of about 11.2 W/m², demonstrating its strong potential for high-efficiency blue energy harvesting.