Polysulfone (PSf) membrane by treated NMP solvents with water forces was studied for pore control and separation of sodium alginate. For the preparation of modified PSf membranes, it was exposed to specific solvents and then treated by water-forces. PSf membrane treated only with NMP could show the separation of sodium alginate above 6 bar since the pore-size was not proper for water transport. However, the membrane treated with NMP and water forces showed the separation performance of sodium alginate from 2 bar. Furthermore, the highest flux (211 L/m2h) and rejection (99.3%) at 8 bar was observed. FT-IR and TGA analysis showed no chemical or physical changes after NMP and exposure to water forces. The modified pore size in polysulfone membrane was confirmed by the porosimeter.