A funnel-shaped Fabry-Perot fiber optic sensor is proposed to satisfy the demand for high-sensitivity measurement of flow velocity and temperature in ocean research. The sensor consists of an aluminum alloy membrane and optical fibers. It detects changes in the cavity length of the Fabry-Perot interferometer based on the deformation and thermal expansion effects of aluminum alloy, which are induced by seawater flow velocity and pressure. These changes shift the interference spectrum, facilitating simultaneous seawater flow velocity and temperature measurements. The flow velocity and temperature sensitivities were obtained through theoretical derivation and finite element simulation. A measurement system was constructed to assess the seawater flow velocity and temperature simultaneously. At a flow velocity of 0.45 m/s, the maximum flow velocity sensitivity reaches -20.13 nm/(m/s), with an average relative error of 4.12% compared to the theoretical sensitivity. The temperature sensitivity is obtained as 10.34 nm/℃, with an average relative error of 4.54% relative to the theoretical sensitivity. A sensitivity matrix dual-parameter demodulation method was employed to measure 10 different seawater flow velocities and temperatures. The results were compared with the measurements obtained via an acoustic Doppler current meter (ADV) and temperature salinity depth meter (CTD). The average relative errors for flow velocity and temperature measurements are 6.07% and 4.20%, respectively. These findings demonstrate that the funnel-shaped Fabry-Perot fiber optic sensor exhibits high sensitivity and can perform dual-parameter measurements. Thus, it is expected to play a substantial role in oceanographic measurements.