Looseness identification of clamped structures based on nonlinear dynamic response is investigated in this paper, according to local nonlinearities in clamped structures. The theoretical solution of the nonlinear free vibration of a cantilever beam is utilized with clamped boundary conditions. The ratio between the second harmonic and the fundamental frequency amplitudesis denoted as r, which is a characteristic quantity to investigate the loosening laws of clamped structures. Aiming at the clamped beam, the nonlinear dynamic response is solved by the contact finite element model. Meanwhile, characteristics of the dynamic responses are analyzed based on structural experiments with different clamping force to verify the method. Numerical and experimental results show that the second harmonic amplitude of the nonlinear response decreases when the clamping force increases, and ralso decreases with the increasing the clamping force, according to the exponent relationship of the power.Therefore, r is an effective characteristic quantity for identifying the state of clamped structure.