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Fiber optic hydrophone is an underwater acoustic signal sensor based on modern optical fiber and optoelectronic technology, which is divided into dry end and wet end. Polyurethane is widely used as the coating material for the wet-end encapsulation of fiber-optic hydrophones. The reliability of the polyurethane packaging directly affects the detection sensitivity, connection reliability and the service life of the detection array of the wet end sensor module. Aiming at the reliability problem of polyurethane encapsulation material for optical fiber hydrophone, a dumbbell-shaped polyurethane sample is prepared in this paper. The sample is placed in the aging environment of hot seawater medium with salinity of 3.5%. The accelerated aging experiment is carried out on the samples. The tensile strength of the polyurethane at different aging stages are tested, and the aging kinetics of the tensile strength of the polyurethane in hot seawater medium at 55°C, 65 °C, 75°C and 85 °C are studied. The results show that the elongation at break of polyurethane at different temperatures decreases exponentially with the aging time, and the mechanical properties of polyurethane decrease faster in the thermal seawater aging test at higher temperatures. The lifetime under operating temperature is predicted by using the Arrhenius equation. The activation energy of polyurethane in seawater is calculated to be 0.65eV, and the service life of the polyurethane in seawater at 25°C is predicted to be 22.81 years.