By analyzing in-depth the connection mode of the impedance-matching balance traction transformer, two practical methods respectively based on symmetrical component and phase component are proposed to calculate the short-circuit current of power grid when short occurs in the secondary side of the transformer. And the formulas for a variety of fault types are given. The comparison of two methods is also proposed. An example using actual grid is given to prove the accuracy and practicability of the two methods. The method based on symmetrical component can be adopted in nowadays calculation programs widely applying in power system based on symmetrical component more conveniently than the method based on phase component and the calculation programs can calculate the short current rapidly by adding special module using symmetrical component when short occurs in the secondary side of the transformer. A software package based on symmetrical component has been applied in the actual project.