Abstract: The present application relates to the technology field of assessment of aging state of oil paper insulation of the large power transformer and quantitative diagnostic, specifically estimating parameters of the circuit through the initial slope of the recovery voltage. It includes an improved mathematical model for solving parameters of the dielectric response equivalent circuit by using the peak of the recovery voltage, the peak time and the initial slope characteristics; transforming the identification of parameters of the equivalent circuit into a mathematical optimization problem, and then solving the mathematical optimization problem using a particle swarm algorithm. The design of the present application can significantly reduce the sampling data of the recovery voltage, and the measured data in the field and the calculated data of different capacity RVM experiments on real transformers have good consistency, which is conducive to the diagnosis of state of oil paper insulation of the transformer.