Abstract: Poly(styrene-co-maleic anhydride) (PSMA) microspheres with smooth surface and narrow size distribution were successfully prepared by membrane emulsificationsolvent evaporation method. The influence factor on the size and size distribution of the prepared microspheres was investigated. Carboxylic groups on the surface of the microspheres were obtained by hydrolyzing the PSMA microspheres in HCl solution. The density of the carboxyl groups on the surface of microspheres was calculated by titration of carboxylic groups. It was found that with the hydrolytic time extending, the concentration of the carboxylic groups on the microbeads surfaces gradually increased and finally reached saturation after 24 h.