Well-defined block copolymers, poly(ethylene glycol) methyl ether-b-poly(styrene) (mPEG-b-PS), in which the PS blocks had different molecular weights, were synthesized by atom-transfer radical polymerization (ATRP) and characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and gel permeation chromatography (GPC). The block copolymers were then used as amphiphilic additives to modify polyethersulfone (PES) hollow fiber membranes to improve the antifouling property. Static contact angle measurement indicated the increase of the membrane surface hydrophilicity, and scanning electron micrograph (SEM) suggested that the modified membranes preserved asymmetric structure. Protein ultrafiltration experiments showed that the antifouling ability of the modified membranes enhanced. After three cycles of BSA solution (1.0 mg/mL) ultrafiltration and three times of hydraulic cleaning, the water flux recovery ratios (F-RR) of the mPEG-b-PS modified membranes were still as high as 85.6%. The hydrophilic modification with mPEG-b-PS copolymers is a good method to improve the antifouling property of PES hollow fiber membranes.