Nanofluidic osmotic energy conversion is an attractive technique to directly convert salinity-gradient energy into electricity. However, the power density of osmotic energy conversion is still relatively low due to the insufficient ion selective transport in nanofluidic channel, limiting its development towards practical applications. We present a nanoparticle enhanced ion selective transport in nanofluidic channel due to the consolidated space charge density via electric double layer of nanoparticles. When a 10% volume fraction of nanoparticles are suspended inside nanofluidic channel, the theoretical osmotic output power under 50-fold salinity-gradient is improved by 43.1%. In addition, the osmotic energy conversion highly depends on the aqueous solution temperature, which significantly affects the ion diffusivity and surface charge density of nanofluidic channel. The temperature of aqueous solution in osmotic energy conversion system could be obviously raised up through immersed nanoparticle based photothermal conversion. Therefore, when a 2.5% volume fraction of nanoparticles are suspended in aqueous solution tanks with 70% photothermal conversion efficiency, the temperature of aqueous solution is enhanced from 293.03 K to 321.56 K. Consequently, the theoretical osmotic output power under 50-fold salinity-gradient is dramatically improved by 485%. Based on theoretical analysis, an experimental verification is carried out to improve graphene oxide membrane based osmotic energy conversion using nanofluid consisted of silver nanoparticles and NaCl aqueous solution under one sun solar illumination. When a 1% mass fraction of silver nanoparticles are immersed in the NaCl aqueous solution to realize photothermal conversion, its equilibrium temperature is raised from 297.2 K to 340.9 K, and the osmotic power density is effectively enhanced from 2.41 W/m2 to 8.43 W/m2 by 249.79% under 50-fold salinity-gradient between artificial seawater (0.5 M NaCl solution) and river water (0.01 M NaCl solution). The current work paves a promising route for consolidating salinity-gradient osmotic energy conversion through suspending nanoparticles under photothermal effect.