Considering typical AC-DC-AC multiple units, CRH2 motor car, in this paper we deduce its parametric equations under sine pulse width modulation, and set up a new nonlinear parameter model. Fractal situation of higher harmonic is analyzed based on chaos theory. Aiming at typical traction substations, calculation and simulation are conducted, and the result is verified based on data from actual wave records, which proves that high-frequency harmonic can lead to system resonance and expands the application of chaos analysis method.