A frequency response measurement method was proposed based on the digitization characteristics of O-E tracking servo system. The testing signal, data acquisition mode, the extraction of amplitude and phase,and hierarchical identification were investigated. The swept-sine signals were generated in the digital motion controller, and the response waveform was sampled and stored on disk automatically. After testing, the stored data were transferred to PC and preprocessed with Matlab software. The phase difference and amplitude ratio were estimated via correlation analysis. Finally, the measurement frequency-domain data were transferred into accurate transfer function with the hierarchical identification method. The experimental results show that the frequency band of the exciting signal is enough for measuring spectra of control systems, and the magnitude phase measurement accuracies of transfer function are ±0.5 dB and ±1° in middle frequency range,respectively, also the parameters of mechanical resonant frequencies can be identified with a bi-quadratic filter in high frequency range. These results confirm that the proposed method is effective and practical.