The present invention discloses a method for producing an epoxidized polymer. The method comprises the steps of: (1) providing a polymer solution containing a polymer having a conjugated diene group; (2) providing a catalyst solution dissolved in the polymer solution, the catalyst solution containing a transition metal ion and a ligand for bonding to the transition metal ion, the transition metal ion being selected from the group consisting of Ti, Mn, V, Mo, W and any combination thereof; and (3) epoxidizing double bonds of the conjugated diene group to produce the epoxidized polymer by providing an epoxidizing agent dissolved in the polymer solution containing the catalyst solution, wherein the epoxidizing agent is meta-chloroperoxybenzoic acid (mCPBA) or analogs thereof.