Nanofluidic salinity gradient power (NSGP), in which power associated with the salinity gradient can be generated with ion-selective nanochannels, has recently received considerable attention due to its high power density and power generation process without any chemical pollutants. Developing a novel nanopore/nanochannel material to attain high performance of NSGP device is essential to develop this next generation, renewable, clean energy. A general design guideline is that the higher the nanochannel’s surface charge density, the greater the NSGP performance. However, the present study will report the first evidence that an optimal surface charge density of a nanochannel is required to generate higher performance of salinity gradient power, which challenge the past understanding so far. To explain what we found in experiments, a rigorous model with considering the deportation and protonation reactions on the channel wall is developed to investigate the power conversion from a salinity gradient with a pH-regulated nanochannel. The result from our modeling is in good consistent with the experimental observation of the unexpected charge-density dependent NSGP behavior. It is also found that the nanochannel with lower surface charge density can have remarkably higher NSGP performance. The reason to cause the interesting and unexpected phenomenon is due to the charge-density dependent ion concentration polarization effect, which yields the decrease in the effective concentration ratio (actual driving force) across the nanochannel. The results are informative for designing relevant apparatus in renewable energy applications.