An in-line distributed optical fiber sensor array based on Sagnac interferometer is developed, and it is used to monitor and locate leakage for the fluid-filled (or gas-filled) pipelines in real-time. The advantage of the sensor is that it can mcrease monitoring distance by using two sensing fibers. The representation of phase change for optic signal caused by leakage is formulated; the monitor principle and the leakage source location method for the detection system are analyzed. The experiment is carried out at 3.990 km and 4.024 km from Faraday rotator mirror 1 and Faraday rotator mirror 2, respectively. The results verify that the system can realize leakage detection and locate leakage source accurately, and the error is less than 1.05%.