The micro-Doppler effect, which provides a new approach for accurate auto radar target recognition, has attracted great research attention in recent years. In this paper the spectrogram of the echo of moving targets with rotating parts in inverse synthetic aperture radar (ISAR) system is analyzed. Then a new method by detecting the sinusoids on the spectrogram is put forward, by which the parameters of sinusoids could be detected fleetly with separating different curves using their periodicity. After that, the actual micro-motion parameters of the target are obtained. A computer simulation is given to illustrate the effectiveness of the proposed method.