Abstract: The polarity reversal（PR） electric field of oil-paper insulation structure was computed by the charge-electric potential finite element method,in which the node charges and electric potentials can be directly obtained.The node charges at every instant could be converted to node charge densities by a numerical method proposed.The normal electric field intensity can be accurately got from the electric charge density of the Dirichlet boundary,which may be helpful to the insulation design.The proposed method was verified by a simple model,which has analytical solutions,and used to analyze the PR process of a typical oil-paper insulation structure of converter transformer.