Abstract: As the electrified high-speed railway develops rapidly, the number of the electric multiple units (EMUs) has increased significantly. However, the high-density EMUs may trigger low-frequency oscillations (LFOs) in the vehicle-grid coupling system. Due to the complex power electronic structure characteristics of the single-phase vehicle-grid coupling system, this paper attempts to apply the Harmonic State Space (HSS) to the modeling of the small-signal impedance of the vehicle-grid coupling system. Firstly, a small-signal impedance model of the vehicle-grid coupling system based on HSS is deduced and then verified via both the Matlab/Simulink platform and the StarSim semi-physical platform. Based on the model deduced and the impedance stability analysis, the mechanism of LFOs caused by multiple EMUs is analyzed and the effect rule of the number of EMUs and grid inductance on the stability of the vehicle-grid coupling system is quantitatively studied. Theoretical analysis and semi-physical verification show that the method proposed in this paper can achieve simple and accurate stability analysis of the vehicle-grid coupling system.