As the main mechanical transmission component of train, the existence of dynamic unbalance of cardan shaft aggravates the vibration of transmission system and easily destroys the core components of transmission system such as bearings and cardan joints. This paper studied the time domain early warning method of dynamic unbalance of cardan shaft by monitoring the abnormal vibration of gearbox, explored the time domain early warning parameters according to the long-term track test, established the time domain early warning model of vibration of cardan shaft-gear box end;established the finite element model of transmission system by using finite element method, and studied the vibration response relationship of cardan shaft with different dynamic unbalance values at different rotational speeds at the gear box end. The results show that the maximum acceleration response of the standard old axle is about 2.8 times of that of the standard new axle at different rotational speeds. The analysis of simulation and measured data shows that the effective value of time-domain early warning of vibration acceleration is 3.72,g, when the train speed is 200 km/h, and 4.97,g, when the train speed is 250 km/h. The rightness of simulation results and warning threshold is verified by bench test.