We develop an in-line distributed optical fiber acoustic sensor array based on Sagnac interferometer use it to measure and locate leaks for the fluid-filled (or gas-filled) pipelines in real time. The advantage of the sensor array is that it can monitor two or many pipelines at the same time. The traditional loop Sagnac interferometer is changed into in-line structure by a Faraday rotator mirror. The representation of phase change caused by leakage is formulated for optic signal; the measuring principle and the leaking source location method for the sensor array are analyzed. An experiment is designed to detect and locate leaks. The results verify that the system can realize leak detection and locate leak accurately, and the error is less than 0.54%.