Extended Debye equivalent circuit of transformer is an important way to analyze insulation aging of transformers. To address the problem of solving the number of polarization branches and the parameters of transformer equivalent circuit, this paper proposes the depolarization current time domain micro-decomposition spectrum method according to the theory of dielectric polarization response. This method first decomposes the components of each sub-line in the depolarization current curve to determine the number of polarization branches and then identifies the equivalent circuit parameters according to the parameters of the sub-spectral lines. Finally, the effectiveness of the proposed method is verified by comparing it with other identification methods. The proposed method provides a reliable and simple method for the construction of the equivalent circuit which accurately reflects the oil-paper insulation condition of transformer and further provides an important basis for the accurate evaluation of transformer insulation.