For salinity gradient energy harvesting, membrane ion selectivity plays an important role, which is often qualitatively analysed via the electric double layer (EDL) overlapping degree in conventional studies. However, the degree of EDL overlapping is hard to be quantitatively evaluated. Here, we systematically analyze the synergy relations between physical vectors that determining the energy conversion process to quantitatively illustrate the EDL overlapping degree and ion selectivity. Three synergy angles are proposed to describe the synergy relations between the ion diffusion and the electrostatic migration driven forces. A synergy degree parameter is further defined, which could offer a quantitative way to analyze the cation transference number under different concentration ratios, channel length, and asymmetric channel geometries. In addition, an alternative way to use large size nanochannels to efficiently harvest the salinity gradient energy is developed by employing nanowire blockers. The inserted nanowire blocker can significantly enlarge the synergy degree parameter, thus to enhance the ion selectivity, upgrade the membrane potential, and bring a significant augment on the electric power and energy conversion efficiency. This study offers a novel insight into quantitatively analyzing the ion selectivity and paves an alternative way for efficiently salinity gradient energy harvesting via large size nanochannels.