In order to evaluate the characteristics of harmonic resonance of traction power supply system and according to the design of traction power supply scheme, the harmonic characteristic simulation model of traction power supply system and harmonic emission model of locomotives (including electric multiple units) are built, and the influence law of harmonic resonance caused by the change of short-circuit capacity of power system and length of feeder cable is studied. The harmonic resonance assessment is applied in the design of traction power supply system, the matching problem of harmonic characteristics between traction power supply system and traction load is discussed, as well as the effect on the power quality. It is proved that the design parameters of traction power supply system have significant effect on harmonic resonance and the optimization design of traction power supply system can reduce even avoid harmonic resonance.