The dielectric response diagnosis technology is an effective method for diagnosing oil-paper insulation state, ensuring the reliability of the transformer’s operation and prolonging its lifespan. In order to further study the relation between the dielectric response characteristics and the insulation state change, indentifying the equivalent circuit parameter is needed. This paper improves the computing method by the spectrum analysis of depolarization current, estimates the time constant of each branch based on the time domain dielectric spectroscopy theory, and then calculates the polarization resistance and capacitance of each branch one by one. The method is easy and can reflect the physical significance. Finally, this method is applied to analyze three transformers and the error between calculated parameter and the original parameter is small, proving the method’s accuracy and feasibility.