Abstract: In this paper, the secondary polarity of PT and CT with load of substation power supply line based on the grid connection of electric railway traction system is studied. First, the V-V connection of transformer of electric railway traction system and the power supply structure of electric locomotive of electric traction system are discussed in detail. Secondly, through the electromagnetic induction law, formula derivation and vector diagram analysis, the PT The relation expression of CT quadratic polarity vector. Finally, the correctness of the expression of the polarity vector relationship is verified by the PT and CT secondary polarity cases of a 220kV substation and a 220kV pull line power supply line in Anshun Power Supply Bureau of Guizhou Power Grid Co., Ltd. It can provide reference and guidance for power supply enterprises to ensure the correct polarity of PT and CT when handling similar loads and ensure the safe, reliable and stable operation of electric railway traction stations.