A new measuring structure based on the principle of Sagnac and Mach-Zehnder mixed distributed fiber-optic interferometer is proposed. The structure can detect pipeline leakage and locate leakage point in real-time. The leakage measuring principle and leakage positioning method for the structure are described, and the wavelet transform algorithm is used to analyze the acquisition of the leakage signals. The pipeline leakage detection experiment is carried out for the length of 3.994 km of the fiber-optic sensor, and the test experiments are repeated 10 times. The experimental results show that the system is stable and the maximum relative positioning error is 1.47%.