In the field of industrial servo control, notch filter is often used to suppress the mechanical resonance, and accurate acquisition of resonance frequency is the primary condition for the successful suppression of resonance. The second-order generalized integrator-frequency-locked loop (SOGI-FLL) is used to analyze the output signal of speed controller in order to detect the resonance frequency. Firstly, the basic structure of SOGI-FLL and the operating principle of frequency-locked loop are introduced, and then the frequency response performance of amplitude-frequency adaptive SOGI-FLL is analyzed. Finally, simulation and experiment show that the amplitude-frequency adaptive SOGI-FLL can measure the resonance frequency accurately and quickly. The measured frequency is used to set the parameters of notch filter, and the mechanical resonance is restrained successfully.