The power electronic traction transformer (PETT) is going to replace the power-frequency transformer of locomotive in the future considering its small volume and light weight. However, the negative damping characteristic of PETT may cause stability problems to train-network system like conventional locomotive. To study this problem, the detailed mathematical model of PETT-based train-network system is firstly established in this paper. Based on the impedance model, the system stability is analyzed using generalized Nyquist stability criterion. Moreover, the influences, including network impedance, parameters of cascaded H-bridge (CHB) and LLC converter, are studied. Lastly, the simulation based on MATLAB/Simulink is conducted to further verify the accuracy and validity of the model, which can give guidance for system parameters optimization.