In order to develop styrene-acrylate copolymer emulsion for cement modifier which had excellent performance and low cost,the pre-emulsification,semi-continuous technology for synthesizing styrene-acrylate copolymer emulsion for cement modifier was discussed with acrylate and styrene as raw materials.The formulation for styrene-acrylate copolymer emulsion and effects of monomer,emulsifier,initiator,chain transfer agent and etc on the emulsion prepared were confirmed.The styrene-acrylate copolymer emulsion had good stability and mechanical properties,which was synthesized with acrylic acid account for 4 %(wt),emulsifiers(anionic and non-ionic composite emulsifier) account for 2.5%(wt) and initiators account for 0.5%(wt) respectively of monomers amount.