A deep research on the contact line (T)-negative feeder(F) short-circuit impedance is made in this paper on the basis of analyzing the advantages and disadvantages of all existing parallel AT power supply system fault location methods . Then, the simulation model of the all parallel AT traction power supply system is established and the characteristic of system impedance under T-F short-circuit condition is simulated and analyzed. The results show that the subsection impedance method basically corresponds to the requirement of engineering application. It also provides some theoretical reference for the T-F short-circuit fault analysis of all parallel AT power supply system.