The special power supply architecture between transformers in traction substation is likely to cause series sympathetic inrush， which may threat the normal operation of traction transformers. The initial inrush caused by the energized transformer running across the system resistance results in the non-periodic fluctuation of voltage at the common node， causing changes in voltage imbalance. A time differential method based algorithm is proposed to distinguish series sympathetic inrush. The algorithm detects the magnitude of the ratio and state of differential current， and regards the moment that the ratio exceeds the threshold as a switch operation. The algorithm recognizes series sympathetic inrush by detecting the time difference between the switching operation and the appearance of differential current. Some special cases are analyzed， such as adding or shedding load suddenly， transferring fault between inside and outside of transformer and appearing internal fault during series sympathetic inrush. The experimental results verify the feasibility and reliability of the proposed algorithm.