Traction power supply system is an important load and one of main harmonic source of power system. Certain simplification is made on traction power supply system modeling when sudying the harmonic transmission characteristc of traction power supply system and the impact of traction power supply system harmonic in power system. Although simplified model has a certain rationality, there will be some errors in the analysis results based on simplified model compared with the actual results. According to the inaccuracy of traction power supply system model, the united model of electric locomotive and traction network and powe system is built by PSCAD/EMTDC software. The characteristics of traction power system resonance and the total harmonic distortion (THD) when the location and number of trains and the length of traction network change are studied. Simulation results show that the united model can more accurately analyze the harmonic characteristics of traction power supply system, and the analysis results are more realistic.