Abstract: In China, the electric locomotives and traction networks have complex power electronic structural characteristics, This situation may cause the harmonic interaction between the vehicle-side converter and the single-phase AC grid, leading to the low-frequency oscillations in the entire vehicle network system, which seriously affects the stability of the network system. Due to the time-varying characteristics of the vehicle network system, taking into account the harmonic coupling process between the vehicle networks, this paper proposes a Harmonic State Space (HSS) small signal modeling method based on the linear time-varying period (LTP) framework in order to have a close study of the vehicle network system. The Multiple-Input Multiple-Output (MIMO) harmonic domain impedance model of the vehicle-side converter is derived firstly. It is revealed that the system has an even multiple of the fundamental frequency coupling process due to the Park transformation in the vehicle-side controller. After that, the established MIMO impedance model is converted into a single-input single-output harmonic domain impedance model that retains all the frequency coupling information, which further simplifies the impedance measurement and impedance-based stability analysis process. Finally, the accuracy of the model is verified through the Matlab/Simulink simulation platform and the StarSim semi- physical platform. Based on the built model, the low- frequency stability of the vehicle-network coupling system is accurately evaluated.