Abstract: The development of high-speed trains has made the safety problem of its key components—bearings increasingly prominent. Existing bearing models are all established under uniform speed conditions and cannot describe the motion state of the system under variable speed conditions. To solve this problem, a dynamic model of the axle box bearing rotor system of a high-speed train under variable speed conditions is established. The model uses angle iteration to calculate the total angle that the rolling elements have rotated in an uneven time, and then determines the spatial position of the rolling elements at any time. Comparison experiments and simulations were carried out under constant speed and variable speed conditions, and they have a good agreement, which verifies the effectiveness of the model. The influence of outer ring fault, inner ring fault, and rolling element fault on the system stability are qualitatively analyzed by the axis trajectory, and the reliability of the analysis results is verified by experiments. The two-dimensional moment invariants are used as a characteristic indicator to quantitatively analyze the influence of three types of faults on system stability. The analysis results show that under uniform speed conditions, the effects of different types of faults on train stability are small. Under variable speed conditions, the outer ring fault has the greatest impact when the angular acceleration is slight, and the rolling element fault has the greatest impact when the angular acceleration is large, but the degree of impact gradually decreases with the size of the fault. Similarly, two-dimensional moment invariants are used to analyze the stability critical state of the rotor system and determine the maximum fault size corresponding to the critical state under different speed conditions and different fault types. The results show that that with the increase of the speed of the bearing inner ring, the maximum size corresponding to different fault types will decrease, and the fault size of the rolling element is mostly the smallest, indicating that the rolling element fault has the greatest impact on the stability of the system.