Abstract: This paper focuses on the stability of the vehicular integrated power system in terms of its island operation, the slow dynamic response of engine, and the complex working conditions. Firstly, the small signal models of each source and loads converter are proposed, based on which the output impedance and loads input impedance are obtained. Then, the instability criterion of small signal of the vehicular integrated power system is established based on an improved impedance criterion. Furthermore, the instability prediction of the system is analyzed from three aspects, such as its engine speed, vehicle speed, and the torque of each driving motor, by using the Bode diagram. Finally, the theoretical analysis is certificated by the Hardware-in-Loop simulation and prototype vehicle tests. The research results indicate that the engine speed should be controlled between 1 900 r/min and 2 000 r/min, and the vehicle speed shall not exceed 100 km/h, when the vehicle is in high-speed mode. The stability of system is heavily influenced by the torque of each driving motor, and the system will be collapsed caused by the torque when it is more than 48 N·m. It is concluded that the proposed instability criterion can precisely predict the system state, and help optimize the control structure and design the assistant system.