Abstract: The dispersion copolymerization process of styrene (St) and acrylonitrile (AN) in polyether was studied by dynamic light scattering，liquid chromatography and UV. The results showed that in the process，in appropriate polymerization condition, the nucleation mechanism was dominated by the agglomeration nucleation of graft polymers, and then the stable dispersion system could be prepared.After the nucleation period,the number of particles kept constant and particles swelled by the action of monomers and grew by the polymerization of monomers in the dispersed phase and kept stable by adsorbing graft polymers formed in the continuous phase.