Abstract: A prototype of Interferometric Synthetic Aperture Sonar(InSAS) for sea trial has been developed.The key techniques are described in detail,which include real-time interferometric signal processing,design of high-stable tow-fish and design of the hydrophone array with high amplitude-phase uniformity.The testing results in lake trial and sea trial show that the InSAS testing system is stable and robust for acquiring the precise Digital Elevation Model(DEM) of seabed with a resolution of 2.5 cm in across direction and 5 cm in along direction,respectively.And the measuring precision of DEM is about 15 cm.