Fiber-optic hydrophone is a new type of sensor which can detect acoustic signal in harsh seawater environment. For some application, the fiber-optic hydrophones should be serving for years or even decades, due to the high cost of placement. Because of multiple fiber-optic hydrophones are made into a hydrophone array by multiplexing generally, thus realize signal detection as a whole. To ensure the detection effect, sufficient number of elements in the array must be working order, otherwise the array may lose efficacy. Slow degradation of fiber-optic hydrophone is inevitable as time goes on. So, it is particularly important to control early failures. In this paper, a research on early failure mechanism of fiber-optic hydrophone has been present. In details, the actual service environment of fiber-optic hydrophone is analyzed firstly, and the weak links of fiber-optic hydrophone which may degrade rapidly are discussed and the early failure mechanisms are also analyzed. Further, the verification test was designed for the possible failure mechanism, and the failure analysis of the samples was carried out by using the advanced failure analysis instrument such as the scanning electron microscope (SEM) and the Fourier transform infrared spectrometry (FTIR). Results show that the bonding process defects is one of main reason for early failure of the fiber-optic hydrophone and the strength of package may degrade rapidly in seawater environment until the early failure occurs. This research will help to improve the process reliability of fiber-optic hydrophones and control the occurrence of early.