Abstract: An FEM model of rotor-bearing system was set up for a single-span-two-disc test rig to obtain such model parameters as the first and second order natural frequency and vibration mode vectors. Based on the assumption of periodic rubbing force in rotor system, the rubbing forces onto each node of rotor system are estimated by modal expansion technique and the sub-harmonic decomposition of residual matrix between faulty and faulty-free vibration signals of rotor system through FFT (fast Fourier transformation). The rubbing position is identified through time serials, frequency spectrum and equivalent magnitude of rubbing force on each node, with corresponding extent of rubbing fault evaluated qualitatively. The method proposed is tested on the single-span-two-disc test rig and proved efficient in precise diagnosis of periodic rubbing force in rotor system, and it also provides a reference for other fault diagnosis of rotor-bearing system.