A series of water-soluble star brush copolymers with four arms consisting of poly(ethylene oxide)(PEO) as the backbone and poly(acrylic acid) as side chains with different grafting density were prepared by sequential anionic copolymerization and then atom transfer radical polymerization (ATRP). The anionic ring-opening copolymerization of ethylene oxide (EO) and ethoxyethyl glycidyl ether (EEGE) was carried out using pentaerythritol and diphenylmethylpotassium (DPMK) as co-initiator, and then ethoxyethyl groups of EEGE units of the copolymers obtained were removed by hydrolysis. The forming four-arm star copolymers of poly(ethylene oxide-co-glycidol) [poly(EO-co-Gly)](4) was esterified by the reaction of pending hydroxyl groups of glycidol units of copolymers with 2-bromoisobutyryl bromide, and the resulting macroinitiators of poly(ethylene oxide-co-2-bromoisobutyryloxyglycidyl ether) [poly(EO-co-BiBGE)](4) were used to initiate the polymerization of t-butyl acrylate by ATRP. Afterward, the tBA ester groups of [poly(EO-co-Gly)-g-PtBA](4) were selectively hydrolyzed by trifluoroacetic acid to obtain the water-soluble [poly(EO-co-Gly)-g-PAA](4). The final product and intermediates were characterized by SEC and NMR in details.