Abstract: A new loosening model is developed for the faults of bearing caps due to loosening, which were often found in actual operation in view of the advantages and disadvantages of various existing pedestal loosening models. With FEM used to simulate the rotor system on which the looseness fault is occurring, the fault characteristics are obtained as follows. When the vibration displacement of equivalent loosening mass is less than the loosening clearance, the system motion is from period-one through period-two to period-four, the amplitude of low-frequency components gradually increases and the elliptical path of axis center of rotor becomes spiral path along with the decreasing of pedestal loosening stiffness. When the vibration displacement of equivalent loosening mass is greater than the maximum loosening clearance, the multiple frequency components and continuous spectra appear in the system and the motion of axis center of rotor is restricted from both sides to form a trapezoid-like moving path with decreasing loosening clearance. These fault characteristics obtained under different conditions are helpful to the diagnosis and analysis of the faults as above.