An interferometric distributed fiber optic leakage detection system based on the mixed interferometer principle of Mach-Zehnder (M-Z) and Sagnac is introduced. The working principle of the system and method for locating leaks are described. Experiments are performed in air and underwater environments. Experiments are carried out at 4025 m, 7043 m and 9050 m from the leakage position to the Faraday rotator mirror. The experiments test the impact of different environments and different leakage positions to the detection system. The experiments are carried out at 7043 m from the leakage position to Faraday rotator mirror and repeated 20 times to test the accuracy of detection system. The results show that in the underwater experiment, the interference signal spectrum strength is greater and band is wider. The farther of leakage points from the Faraday rotator mirror, the greater absolute error of the experimental results. The system can detect and locate the slight leakage signal of the gas pressure within the pipeline at 0.5 MPa, with leakage aperture of 2.5 mm, and average relative positioning error is 1.44%.