In recent years, there exists some low frequency oscillation (LFO) events in high speed railway when multiple electric motor units are accessed to the traction network. To better deal with the problem, this paper regards the vehicle-grid system as a cascade system. The mathematical small-signal model of vehicle-grid system is established first. Then the norm stability criterion is adopted to analyze the system's stability considering the impedance specification of vehicles and grid. The simulation results show that the LFO can be analyzed with the method presented in this paper. In addition, an effective improvement measure is recommended to restrain the LFO of vehicle-grid system.