Abstract: Through the analysis of operating theory of AC-DC-AC locomotive in traction power supply system, this paper established a simplified model of CRH380B and active power, power factorand DC side voltage act as important factors. The united simulation platform of traction loads of locomotive, traction grid and power system were built in PSCAD/EMTDC IDE. The paper analyzed the effect of running position of locomotive in traction power supply system for the voltage level of the catenary when CRH380B was in the stable operation, and the effect of locomotive position on the grid and the number of locomotive for current harmonic distortion. Simulation results show that the CRH380 model and the united model built in the paper are simple and reasonable. And they could be able to analyze locomotive current harmonic characteristic, the catenary voltage distribution of traction power supply system and the total harmonic distortion characteristics of the grid.