With voltage low frequency oscillation of high-speed railway traction network and EMU coupling system, many EMU of different places appear traction blockade. Mechanism of this phenomenon is currently in its early stages. Firstly, the transient energy balance equation of traction network and EMU coupled system are derived. Secondly, the open-loop stability of the traction network and EMU system is determined and the factors of voltage oscillations are analyzed with the small gain theorem. Thirdly, the all-parallel autotransformer traction supply model is precisely built by using the 10-conductor chain circuit and the CRH3 EMU electrical model is established based on the transient current control. On this basis, the interconnected system of the traction network and EMU is implemented, and voltage oscillation phenomenon is reproduced. In addition, the frequency of the oscillating voltage is accurately extracted by using FFT method, wavelet algorithm and Prony method.