In the large disk?shaft rotating machinery with interference fit, high input speed of the shaft may result in the looseness between the disk and the shaft, and affect the vibration characteristics of the system. A vibration model of the disk?shaft system with looseness rotating part is built to analyze the rotor dynamics. Normal force and contact pressure in the model are considered as bi?linear form. It is found that when the input speed of the shaft exceeds a critical speed, the vibration of the disk?shaft system diverges in the first linear stage and converges in the second linear stage, resulting in the speed difference between the disk and the shaft, and the vibration of the system becomes a superposition of two different frequencies. The second linear stage represents that a gap occurs between the disk and the shaft, indicating that the loose happened.