Abstract: In the paper, it is described that the behavior of the mach-zehnder type single mode fiber-optic hydrophone with water-proof coating on the signal fiber and carried one phase drift compensator. This paper derived the single mode condition for step type optical fiber. Because the fiber has the polyfluortetraethylene coating and the property of singlemode, it can keep a good operating Performance even though it is emmersed in the sea-water. Some results for the principal character of this type fiber-optic hydrophone have been evaluated theoretically, and the circuit of phase drift compensater has been analysed.It may be develope one kind new type sonar(opticalsonal).