Aiming at the transient zero-mode characteristics of single-phase ground fault in neutral point ineffectively grounding system, a method of fault line selection is presented based on the Root Mean Square (RMS) ratio of modulus maximum. This method analyzes the singularity differences of transient current between fault line and non-fault line by using wavelet packet theory and singularity detection theory. It calculates the modulus maximum RMS in line selection band and determines the fault line by calculating the ratio of modulus maximum RMS. Some factors that might influence the effect of fault line selection in the actual application are taken into account. Results and a lot of EMTP simulation show that the proposed method is of high sensitivity and accuracy and strong adaptive ability.