In order to accurately identify the moment of inertia, the nonlinear torsion pendulum model is established by considering nonlinear damping and the nonlinear restoring force. An identification method for the moment of inertia based on the Hilbert transform is presented. First, the envelope signal and instantaneous angular frequency are computed by the Hilbert transform of the angular displacement signal. Then, the instantaneous undamped natural frequency of the torsion pendulum is calculated by the Hilbert transform. Finally, the test object′s moment of inertia is identified by the torsion pendulum with an air-hovered turntable. The experimental results show that the relative error of the moment of inertia is less than 1%, which validates the identification method.