Salinity power is a promising renewable energy with a large amount, which can be tapped by pressure retarded osmosis(PRO), however, efficient membranes are in lack. It was reported subtrates of asymmetrical reverse osmosis(RO) membranes had severe concentration polarizations in PRO process, degrading performance greatly and limiting PRO application. Based on solute transportation equations, the paper presented a common mass transfer resistance model of osmosis process, and disclosed essential differences between PRO and RO by the comparison of resistances. In PRO water permeates through membranes against solute diffusion, which facilitates solute to accumulate in porous subtrate, as a result, subtrate mass transfer resistance is big, water flux and power density is small, especially in the case of trivial subtrate mass transfer coefficient. Commercial RO membrane CA-3000 was studied for PRO application, whose subtrate resistance was found much bigger in PRO, taking the majority of total resistance. Subtrate was optimized for PRO process and membrane performance was projected. When subtrate mass transfer coefficient was improved to 4×10-6 m/s, power density of 4.75 W/m2 was obtained with average sea water, which approached the threshold of commercial salinity power exploitation.