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This paper reports a novel design of a MEMS type underwater acoustic sensor using SOI (Silicon in Insulator) wafer as the starting material. The structure layer of the SOI wafer provides a uniform membrane thickness which is perfect for sensing structure. By chemical vapor deposition thin layers of silicon nitride and poly-silicon, the sensing membrane and piezo-resistive material are patterned and etched using lithography and dry etching processes, respectively. After the gold wire is patterned, the entire back side of the sensing membrane is etched in order to create the cavity. The waterproof of the hydrophone is completed by deposition a thin layer of Parylene polymer material. Another fabrication technique starting from a 6" single-crystalline silicon wafer by controlled etching method will also present. The sensitivity comparison between the MEMS type acoustic hydrophone and the piezoelectric one is made, and some conclusions are drawn. We have proven that the controlled etching process can create the sensing membrane without difficulties. Currently, the research project is still undergoing, part of the results will be shown at the conference.