A new method for the functional modification of polyethersulfone (PES) is described in this paper in which PES was sulfonated by chlorosulfonic acid firstly and then chlorinated by phosphorus pentachloride. Thereby, chlorosulfonic groups (-SO2Cl) were introduced in PES successfully, and the degree of the chlorosulfonation was about 15%. Since the chlorosulfonic group is reactive enough towards the amino group (-NH2), the conformation of chlorosulfonic-based covalent bonding functional route could further extend the PES application area with versatile functionality. In this article, hydroxyl (-OH), carboxyl (-COOH), amino (-NH2) and methyl (-CH3) groups were grafted onto a PES matrix through the reaction of chlorosulfonic groups and the amino groups. The functional groups have effects on protein adsorption and cell behavior. The membrane modified with an amino group was in favor of cell adhesion and proliferation, and the membrane modified with hydroxyl and methyl did not so well as the membrane modified with an amino group, which will give guidance for biomaterial modification.