In view of the fact that the existing phase mode transformation matrix can not satisfy the decoupling calculation of the multi-line system of the traction network with the line frequency parameters, this paper pre- sented a method of calculating the phase mode transformation matrix, i.e. the approximate iteration of the eigen- vector was realized by the LM (Levenberg-Marquardt) algorithm based on the mathematical relationship between the eigenvector and the eigenvalue. The research results show that the smooth continuity of the phase mode transformation matrix in the frequency domain can be guaranteed, and the feasibility and accuracy of the method are verified through the phase velocity obtained by other phase matrix transformation methods.