The actual load rate of railway traction transformer is mostly below 0.3, far below the normal load rate standard of 0.5 and above, which is called low load operation condition. By analyzing the overload capacity of four typical traction load curves under normal load rate and low load rate, it was proved that traction transformers had higher overload capacity under low load rate operation, but for different load curves, their overload capacity varies greatly. Curve 1 and curve 3 could be increased to 1.25 to 1.56 times of normal overload capacity, while curve 2 and curve 4 could not obviously improved, which provides a reference basis for reasonable selection of installed capacity of traction transformers.