Abstract: It is difficult to calculate accurately the short-circuit of DC power supply system in urban rail trains. Based on the external characteristic curve of multiple polyline rectifier unit, the 24 pulse rectifier equivalent model is improved. Then, an equivalent circuit model is built and the mesh current equation is formulated considering all of the rectifiers and transition resistance. In order to get the working range and steady short-circuit current of rectifier unit at each traction substation, the algorithm based on Gauss-Seidel iteration is proposed and implemented by MFC program. Finally, some sections on Xi′an Metro Line 3 are taken as the example, the steady state short-circuit current is calculated, and the validity of the algorithm is verified.