This paper reports a reciprocal Mach-Zehnder interferometer in a dual Sagnac ring configuration, and one of two arms in Mach-Zehnder interferometer is used for underwater optic fiber hydrophone. Two Mach-Zehnder interferometers operated in opposite sense are simultaneously induced from the underwater acoustic wave and the PZT phase modulation. Thus, at the output, four interferometric beams occur such that the intensity of interferometric beams will enhance. In general, one stage of a double Mach-Zehnder interferometer is just used as a sensor, another one for compensation. Specially, this paper presents two stages of a double Mach-Zehnder interferometer sense the measurand at the same time due to the topology structure of Sagnac ring pairs. In the other words, the characteristic is to win the affection of two beauties at the same time. Two rings in the same direction make a Mach-Zehnder interferometer. Besides, the inner ring in the clockwise direction and the outer one in the counterclockwise direction also make a Mach-Zehnder interferometer. So as to the unbalanced arms of the Mach-Zehnder interferometer, the underwater acoustic wave induces the light beam phase difference in optic fiber hydrophone interrogator to demodulate the acoustic wave signal by PGC circuit.