In China， current transformers（TAs） are just installed at A-phase and C-phase in most non-solid earthed distribution networks. Most of the existing line-selection methods for single phase grounding fault are based on zero-sequence current. How to detect and separate the grounding-faulty line with two phases' TAs， has been a very difficult technical problem which affects practical operation. Through the analysis of transient currents in frequency domain and time domain， the relationship of A-phase， C-phase transient currents and B-phase transient current has been founded out. A method to produce B-phase transient current with A-phase and C-phase transient currents is proposed when A or C phase is earthed， and hence the exact zero-sequence current can be obtained. The results of simulation demonstrate the correctness of this method.