The low-frequency oscillation phenomenon had been frequently occurred in China electrified railways, which caused the traction locked of the train and the protection tripped of power supply system as well as other accidents. The mechanism of this lowfrequency oscillation in train-traction network system and its influential factors were fully investigated through both simulation and theoretical studies. The impedance modeling method for converter was presented. Based on the stability criterion of impedance rate, the influence of different available factors had been detailed studied, such as the number of trains, equivalent impedance of traction network and the control parameters of onboard rectifier on the low-frequency oscillation of train-traction network system. The results of the theoretical analysis in impedance model were verified by the detailed time domain simulations, which provided a reference for suppressing the low-frequency oscillation in train-traction network systems.