Design of support layer structures for asymmetric thin film composite membranes has drawn keen attention to improve the power density for salinity gradient power based on pressure-retarded osmosis. This study has interests on electrospun nanofiber-based support layers, and the effects of its open pore sizes are attractively stated. To control the open pore size, a counter charge deposition method was introduced. To retain the open pore size, all the nanofibers were interconnected by a post co-solvent treatment technology. For a thin film composite membrane, an interfacial polymerization was used to fabricate a polyamide active layer on the electrospun nanofiber-based support layers. It was found that although the maximum power density achieved with an open pore size of 2.4 μm2 was 0.14 W/m2, it increased significantly up to 9.5 W/m2 when the pore size was reduced to 0.65 μm2. The cause is the salt flux which increases with increasing the open pore sizes under applied pressures.