By taking a twin diaphragm box as the sensing component for underwater pressure,a micrometric optical fiber displacement sensor was employed to inspect deformation of diaphragm box with water pressure changing.The signal processing circuit mainly includes signal pre-amplifier,BPF,RMS converting circuit,and 16 Bit MSP430 with ultralow-power consumption.The experimental results indicate that the change of water level from soutis 0 mm to 181 mm can be detected with the resolution of 0.7 mm.The resolution can be improved by increasing the amount of the diaphragm boxes.