Abstract: To verify the feasibility of Interferometric Synthetic Aperture Sonar (InSAS) in free tow mode other than rail mode, an experimental InSAS system has been developed, and a set of experimentations has also been conducted. The primary purpose of the experimentation is to confirm the validity of the image formation algorithm of multi-subarray synthetic aperture sonar proposed by us, platform and the phase unwrapping algorithms. An interferometic synthetic aperture image is successfully formatted by experimental results, and clearly shows the effectiveness of the InSAS system. It also shows the resolution ability of 5 cm and 10 cm in range and in azimuth respectively. The accuracy of bathymetry is better than International Hydrographic Organization (IHO) standard.