Abstract: Terpolymers of ChMI/MMA/St were synthesized by semi-batch emulsion polymerization.The thermal properties,mechanical properties and rheological behavior were evaluated.The results shown the decomposition temperature of terpolymers increased with the increasing of St feed content.The apparent viscosity of terpolymer in melt decrease with St feed content.The terpolymer was used as thermo-resistant agent and blended with PVC.The thermal properties,mechanical properties and rheological behavior of blend were also evaluated.Results show the glass transition temperature and Vicat soft temperature of the blends increased with terpolymer content increasing.The tensile strength increase with terpolymer content increasing while impact strength decrease.