This article is about some results of the research for increasing the phase-shift sensitivity of fiber-optic hydrophone. Many factors that affect the sensitivity are discussed, including the structure of the hydrophone sensor, the substrate of the fiber and the composite structure of the fiber. The framework of the hydrophone sensor is enwound by the fiber of the reference arm of the interferometer; out of this an inner lining is used with the fiber of the signal arm enwound on it. The fiber has a composite structure. From inner to exterior there are the core, the cladding and the substrate. The thickness of the substrate has an effect on the phase-shift sensitivity of the hydrophone. Outside the sensor we spread a kind of elastic material that is sound non-transparent. This material can protect the fiber of the signal arm. It can also be regarded as the substrate of the fiber, so the thickness of the material can affect the phase-shift sensitivity of the hydrophone. The theory analysis and the experiment measurement have been made in detail in this article. And the result is that 1mm of Teflon can raise the phase-shift sensitivity by 10dBrad/Pa.