Carboxylic polyethersulfone (CPES) was synthesized through a facile acetylation reaction followed by an oxidation treatment and used as a modifier in support layer composition of forward osmosis (FO) membrane. The hydrophilic CPES polymer could ameliorate thin film composite (TFC) membrane performance in water permeation. The inferior sponge-like structure of the pristine PES substrate was changed to appropriate long finger-like one, when the CPES modified polymer dope solution was casted in a basic coagulation bath. In such operational condition, attached carboxylic functional groups were almost deprotonated and the electrostatic repulsion forces among them resulted in the formation of a substrate with open pores and less dense morphology. To prove pH-responsive characteristics of the CPES blended membranes, pH of coagulation bath was switched from 3 to 12. By doing so, water flux drastically increased from 14.23 to 28.03 LMH and from 27.80 to 46.17 LMH in FO mode (active layer facing feed water) and PRO mode (active layer facing draw solution), respectively. Further studies showed that such pH treatments could also lower the unfavorable structure parameter (S) from 1423.54 to 653.94 mu m. The pH-reversibility of the obtained membranes were evaluated using different draw solution pH values. In an acidic draw solution (pH = 3), the protonated carboxylic functional groups started shrinking and thus could open the substrate's pores, which facilitate rapid water transportation. This investigation may open up a new class of pH-responsive and pH-reversible material for promising FO process, in which permeability and selectivity of a FO membrane could be readily controlled through fluctuation in pH of coagulation bath or draw solution.