The PB g PSG latex was synthesized by semicontinueous emulsion grafting polymerization. Twain 20 was added to latex with 3%, 5% and 7% of solid mass of latex. PB g PSG latex with Twain 20 was added to cement mortars to prepare PB g PSG modified cement mortar. The effects of use level of Twain 20 and polymer cement ratio(by mass) on the fluidity, compressive and flexural strength, water retention rate and absorbing water speed of PB g PSG latex modified cement mortar were studied. Microstructure of modified cement mortar was observed by SEM. The stability of Twain 20 to PB g PSG latex was analyzed. The results show that the fluidity of modified cement mortar is increased with increasing use level of Twain 20 or polymer cement ratio. Absorbing water speed is decreased at first, and then is increased with increasing use level of Twain 20. PB g PSG latexes with 3%，5% and 7% Twain 20 increase water retention rate of cement mortar, and the maximum is 9922%(by mass). The compressive strength of modified cement mortars is decreased with increasing polymer cement ratio, and is lower than that of control mortar. The flexural strength of modified cement mortars is basically higher than that of control mortar. The maximum of flexural strength of modified cement mortar is 945MPa at 10% of polymer cement ratio. Latex plays the bonding and bridging role to cement hydrates, and forms interpenetrating network structure, so as to improve the performance of cement mortar. Due to Twain 20, a thick hydration layer will be formed by the latex particle surface, hindering the latex particle close to each other and coagulation, with the result that the PB g PSG latex is more stable.