This paper theoretical analysesSpower grid three phase unbalancedSand harmonics in caseSof theStraction power supply system common transformer wiring, accounted for using the arithmetic power factor, vector power factor and effective power factor three definitions of methods respectively, discusses the three phase unbalanced and nonlinear of traction power supply system power factor computation. Theoretical analysis and calculation examples shows IEEE STD.1459-2010 trial standards recommended for using effective power factor can accurately calculate the three phase unbalanced and nonlinear system power factor, provide a scientific basis for the correct assessment ofSenergy measurement.