Abstract: During the toughening process in which the thermoset epoxy resin was toughened by the thermoplastic polyethersulfone(PES), the phase separation process could be controlled to optimize the phase morphology of the PES-epoxy resin blend systems, for the sake of improving the impact strength of thermoset epoxy resin. In the phase separation processes of PES-epoxy blend systems, in which both the stress relaxation phenomenon of PES and the curing reaction phenomenon of epoxy resin were considered, the viscoelastic model was used to describe the evolution process of micro-structure, then the mechanism and dynamic process of the phase separation processes were revealed, and the influences of composites material and processing parameters, such as PES content, PES molecular weight, dynamic asymmetry degree between PES and epoxy resin, curing conditions, on the phase morphology evolution of blend systems were analyzed. And then the basis for the optimization of phase morphology of the PES-epoxy systems was established.