Mussel-inspired polymer/dopamine coatings often require several separated steps including the grafting of dopamine onto macromolecules and the deposition of mussel-inspired macromolecules. Herein, we attempt to investigate the direct conjugation between catechol and O-sulfated chitosan to fabricate coatings onto polyethersulfone (PES) ultrafiltration membranes; with the addition of hexanediamine, the coating compositions and membrane performances were further adjusted. The conjugation was confirmed by 1H nuclear magnetic resonance (1H NMR) and the coatings were characterized by X-ray photoelectron spectroscopy (XPS). The as-prepared O-sulfated chitosan coatings could enhance the protein antifouling property of PES membranes. Subsequently, the coatings could be further sulfated to obtain N,O-sulfated chitosan coatings, which confer PES membranes with richer functionality. The membranes with N,O-sulfated chitosan coatings exhibited high anticoagulant activity, low platelet activation, low complement activation, as well as low hemolysis ratios, and were favourable for cell proliferation, demonstrating excellent biocompatibility. Meanwhile, the fluxes of the membranes exhibited durable sensitivity to ionic-strength, which also reflected the stability against electrolyte solutions. Besides, the coatings were proven to have applicability on different polymeric membranes. Hence, it was believed that this approach could provide another option to endow membranes with excellent biocompatibility in various biological fields.