The return system is an important component of the traction power supply system. Under the direct power supply mode, the traction return system consists of multiple circuits with a complex structure. If the return is not smooth, it may cause the supporting fittings to melt and burn, seriously threatening the safety of railway operation. This study aims to analyze the mechanism of support fittings melting and burning caused by traction return system faults under direct power supply mode, clarify its key influencing factors, and provide theoretical basis for fault diagnosis and troubleshooting of traction return system. Firstly, an in-depth analysis was conducted on the current distribution characteristics in the traction return system, and a simulation model of the traction power supply return system based on CDEGS was established; Then, the current and potential distribution characteristics were simulated under typical fault conditions such as normal working conditions, ungrounded contact wire pillars, and ungrounded return lines; Further comparative analysis was conducted on the influence of different soil resistivity and grounding resistance on the distribution of system current, and the reasons for the melting and burning of pillar fittings were summarized. The simulation results show that under normal operating conditions, the maximum return ratios of the return line and the through ground line are 37.00% and 19.00% respectively; When the return wire is not grounded, the proportion of return wire and through ground wire return is 47.34% and 10.10% respectively. At this time, some current flows into the through ground wire through the pillar, and the maximum current passing through the pillar can reach 74.73 A. There is a maximum voltage difference of 40 V between the return wire and the pillar. When the return wire is not grounded, there is a voltage difference between the return wire, pillar, and through ground wire, and a large current flows through the supporting fittings on the pillar. The combined effect of the two causes the supporting fittings to heat up and melt. In actual operation, it is necessary to strengthen the investigation of ungrounded return lines to avoid the occurrence of such faults.