Traditional symmetrical component method only works under the condition of symmetrical parameters of transposed transmission lines. The error can’t be ignored if the transmission lines are un-transposed single-circuit transmission lines. The modal analysis can be used for better results. The key of the modal analysis is to get a phase-mode transformation matrix. This paper considers the effect of the existence of the earth wire and uses perturbation theory to get phase mode transformation matrix of different precisions by building a perturbation made by un-transpose. Applying the obtained matrix to the theory of modal analysis, it finds that the calculated results have small error compared with the real results, verifying the feasibility of the method.