The equivalent model analysis is an effective method to indirectly assess the aging state of oilpaper insulation, and it is essential and difficult to construct the equivalent model and identify model parameters of the oil paper insulation system. In view of the shortcomings of the extended Debye model, the mixed polarization equivalent model of the oilpaper insulation with the interface polarization branch is introduced to describe the actual polarization process. At the same time, the parameter identification method of dielectric spectrum model in frequency domain for oilpaper insulation mixed polarization process is studied. Firstly, the relationship between frequency spectrum and circuit parameters is derived based on a new polarization equivalent circuit. Secondly, the objective function for solving multivariate equations is established, and then the parameters are solved by ACPSO algorithm. Finally, the feasibility and accuracy of the method for identifying model parameters are verified by measured data. Results show that the mixed equivalent model considering the interface polarization can reflect the relaxation process of oilpaper insulation more truly, and it is more consistent with the actual situation of transformer insulation polarization. Compared with the identification method based on timedomain dielectric spectroscopy, the identification method of the equivalent model of oilpaper insulation based on FDS is simple, accurate and reliable.