Real-time hybrid test has been applied to the performance test of high-speed train running on the bridge in recent year. In order to avoid the damage of specimens or loading systems caused by instability, it is necessary to study the stability of real-time hybrid test on traveling train-bridge system. The time-varying characteristic of the traveling train-bridge system poses challenges to the stability analysis of real-time hybrid test. Therefore, it is necessary to develop suitable stability prediction methods for the time-varying system. Firstly, the time-varying discrete state space equation of the real-time hybrid test on traveling vehicle-bridge system was established, which can accurately describe the changes of all state quantities of the test system over time. Then a stability criterion based on the spectral radius of the cumulative state transition matrix was proposed, and then by combining the stability criterion with the dichotomy method, a relative stability prediction method for the time-varying real-time hybrid test was developed. A serial of practical real-time hybrid tests on traveling vehicle-bridge system was conducted based on a shaking table. The results show that the critical stability obtained by the practical tests was in good agreement with the predicted results based on the developed stability prediction method. The developed method can accurately predict the stability of RTHT on vehicle-bridge coupled system.