By using an aqueous H2O2 solution and the reaction-controlled phase-transfer catalyst CPC-PW4O16, polybutadiene (PBD) was epoxidized quantitatively and efficiently up to an epoxy ratio of 20 mol %. During the epoxidation, the catalyst first interacted with H2O2 to generate an amphiphilic intermediate, which made the reaction mixture become a two-phase emulsion. The epoxidation of PBD occurred in the interface of two phases. With the consumption of H2O2, the catalyst was self-precipitated out of the emulsion, thereby becoming recyclable. The starting material of PBD can be either the commercial block rubber or the original rubber solution directly from the polymerization reactor. When using the latter, many processes consuming time and energy in preparation of PBD solid rubber and in preparation of EPBD could be removed. The recycle test indicated that the activity of catalyst still remained high, compared with the fresh one.