Nanofluidic reverse electrodialysis offers an alternative way to harvest the widely-existing salinity gradient energy. In this study, we investigate the impacts of surface hydrodynamic slip modification on the ionic current rectification and salinity gradient energy conversion via a conical nanopore by thermodynamic analysis and numerical simulation. Results reveal that in the configuration where hydrodynamic slip modification is employed on the surface near the tip side, a small modification fraction contributes to the ionic current rectification due to significantly enhanced ion enrichment, while a larger hydrodynamic slip modification fraction goes against the ionic current rectification for deteriorated ion enhancement and induced counter-electric field concentration gradient near the base side. For energy conversion, at low concentration ratios, a large modification fraction brings an obvious augment on the electric power. However, it decreases energy conversion efficiency. At a concentration ratio of 100-fold, the electric power is increased by 60.7%, when half of the nanopore wall is modified into hydrodynamic slip. And the energy conversion efficiency is decreased by 6.41%. These findings shed light on the role of hydrodynamic slip modification on the ion transportation and salinity gradient energy conversion, and help developing high performance nanofluidic systems via hydrodynamic slip modifications.