Abstract: The fixed point method is adopted to analyze the nonlinear electric field under the AC-DC hybrid voltage,which contains many high order harmonics,and the formula of selecting fixed-point conductivity was proposed.For the stiffness matrix is independent of the current iteration’s solution,the finite element method（FEM） in frequency domain can be used to obtain the steady periodic results effectively.To verify the proposed algorithm,a typical oil-paper insulation structure of converter transformer under the superimposed AC-DC voltage was calculated by different methods and the results show that the proposed method is suitable to analyze the nonlinear electric fields under AC-DC hybrid voltage of actual converter transformer and other periodic non-sinusoidal electric problems.