The main purpose of the optical fiber sensing technology is to detect perturbation of physical fields. By means of some demodulating scheme, we can extract the real signal from those light beams which modified by physical fields. In the thesis, we proposed a configuration of modified Sagnac Interferometer as a sensing system. The optical sensing and demodulation system are exploited separately. Next, we study the advantages and disadvantages of the configuration. Besides, we are also measured the sensitivity and dynamic range. The sensing system used a low coherence light source to reduce cost. This system also improves the shortage of a Sagnac Interferometer which has a blind point in the middle position. In addition, the structure is easily implemented and can detect weak signal in a high noisy water environment. For matching the main structure, we make many kinds of sensing heads for detecting signals under water. We also use the mathematical model as the base of the theory. The dynamic range is 40 dB and the sensitivity is -231.47 dB re V/uPa.