Hydrophilic surface modification of poly(phthalazinone ether sulfone ketone) (PPESK) porous membranes was achieved via surface-initiated atom transfer radical polymerization (ATRP) in aqueous medium. Prior to ATRP, chloromethyl groups were introduced onto PPESK main chains by chloromethylation. Chloromethylated PPESK (CMPPESK) was fabricated into porous membrane through phase inversion technique. Hydrophilic poly(poly(ethylene glycol) methyl ether methacrylate) (P(PEGMA)) brushes were grafted from CMPPESK membrane under the initiation of benzyl chloride groups on membrane surface. The results of Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) confirmed the grafting of P(PEGMA) chains. Water contact angle measurements and protein adsorption experiments suggested that the hydrophilicity and anti-fouling ability of PPESK membrane were remarkably improved after the grafting of P(PEGMA) brushes. The addition of small amount of water in the reaction solvent apparently accelerated the progress of the grafting reaction. The use of CuCl2 in the catalyst system promoted the controllability of the ATRP reaction.