To avoid the impact of oilpaper insulation transformers’ aging on its equivalent circuit, the recovery voltage differential spectrum solution method is proposed to determine the number of equivalent circuit’s polarization branches by analyzing the functional relation between the recovery voltage spectrum line and the parameters of the equivalent circuit. The proposed method solves the longtime knotty problem in determining the number of polarized branches of the equivalent circuit, laying a solid foundation for diagnosing the insulation state of the transformer accurately. Case studies show that the number of polarization branches of the equivalent circuit can be determined accurately and the insulation aging condition of the transformer is reflected more accurately by using the recovery voltage differential analysis method. Moreover, the number of the differential spectrum lines and polarization branches becomes larger when the transformer insulation aging becomes worse.