AbstractInadequate mass transportation of semipermeable membranes causes poor osmotic energy conversion from salinity‐gradient. Here, the lamellar graphene oxide membranes (GOMs) constructed with numerous fusiform‐like nanochannels, that are pre‐filled with negatively charged polyanion electrolytes, to both enhance the ion permeability and ion selectivity of the membrane for energy harvest from the salinty gradient, were developed. The as‐prepared membrane achieved the maximum output power density of ∼4.94 W m−2 under a 50 fold salinity gradient, which is 3.5 fold higher than that of pristine GOM. The enhancement could be ascribed to the synergistic impact of the expanded nanochannels and the enhanced space charge density. Via feeding with the artificial salinity water and monovalent cation electrolytes, the system could realise the power output up to 14.7 W m−2 and 34.1 W m−2, respectively. Overall, this material design strategy could provide an alternative concept to effectively enhance ion transport of other two‐dimensional (2D) membranes for specific purposes.