Styrene/maleic anhydride copolymer-g-PEG400 (SMA-g-PEG400) hydrogels were synthesized and characterized by FT-IR. Influence of environmental pH value, temperature and ionic strength on swelling performance of the hydrogels had been investigated. Especially,its dynamic swelling mechanism had been researched. The result showed that these hydrogels made a quick volume response to the changing of pH value, temperature and ionic strength, which showed some features of intelligent gel. The paper chose erythromycin as the model medicines and investigated the drug release behavior of SMA-g-PEG400 at different medium ,pH=1 and pH=6.8 respectively. Result showed the cumulative release ratio at pH=6.8 was higher than at pH=1. Based on the result of drug release , conclusion could be arrived at: if the crosslinking point density and esterification degree of SMA-g-PEG400 be controlled properly, the sustained-release hydrogel SMA-g-PEG400 possessing proper release speed which were demanded by medicine industry could be gained.