Abstract: The paper proposes a method that is based on a combined time-frequency and space-time processing of the data from the dynamic lake trials on synthetic aperture sonar (SAS). After range compression, we used time-frequency processing to improve as much as possible the signal-to-reverberation ratio. The time-frequency processing is the kernel of the method, it extracts the characteristics of targets skillfully. The space-time processing is incoherent accumulate processing. The proposed method can detect small targets more clearly than SAS do, but the images of small targets are poorer than that of SAS. CTFSD method is a new technique to detect both proud and buried targets.