Nanofluidic salinity gradient power (SGP) harvesting has attracted more attention due to its abundant source and easy-to-implement nature, but its applicability is hampered by the high resistance and undesirable ion selectivity of the commercial ion-exchange membranes. The rational design and optimization of advanced membrane architectures are therefore urgently needed, which require a deep understanding of the relationship between ion transport and porous structure. Herein, a regular network model is defined to mimic the nanoporous membrane, allowing us to study the influence of porous structure on the SGP conversion using the Poisson-Nernst-Planck equations. We find that the slight change of inner porous structure can substantially affect the concentration polarization at the membrane-solution interface, which is dominated by the nanopores along the direct transport direction. The SGP conversion can be effectively improved by matching the local pore structure to electric double layer (EDL) structure. The SGP generation is more sensitive to the surface charge density at high salinity gradients. Our results highlight the significance of the precision regulation of porous structure in the SGP conversion, providing theoretical guidance to the design of nanoporous membranes for highly effective SGP harvesting.