Thermoplastic resins have been demonstrated to enhance the toughness of epoxy resins (EP) without compromising their heat resistance and modulus. Moreover, thermoplastic/EP composites exhibit excellent electrical properties. As a filler, however, thermoplastic resins often increase the viscosity of the castables, which has always plagued its application in the field of power equipment casting. In this paper, 1,4-butanediol diglycidyl ether (BDDE) was utilized to dilute the hydroxyl-terminated polyethersulfone (PES)/EP system with the objective of enhancing its processability and enabling its application in the field of casting of electrical equipment. It was found that the introduction of this reactive diluent reduced the viscosity of the castables. When 5 wt% diluent was added, the system viscosity was reduced by 47.6%. Concurrently, the glass transition temperature (Tg), tensile strength, and volume resistivity of the epoxy resin composites remained largely unaltered. These findings offer valuable insights for the further exploration of the applications of thermoplastic resins.