Abstract: The conventional range-Doppler algorithm in Inverse Synthetic Aperture Sonar (ISAS) is based on model of uniform motion target. When target moves irregularly, the method may be invalidated because the Doppler effect of each scatterer and imaging projection plane may change with time. A new range-instantaneous Doppler imaging method based on time-frequency distribution is introduced in this paper. It is proved by experimented data that this method can be used in the underwater imaging.