The pipeline leakage detecting and locating system based on Sagnac interferometer and distributed optical fiber technology has many advantages,such as the long monitoring range and the high precision in fault locating. In the prototype,the non-sensing fiber portion of the Sagnac ring must be shielded to avoid disturbance,which will restrict the application in actual monitoring system. To address this problem,a new optical configuration was put forward in this paper. It adopted one optical fiber line as sensing part instead of the optical fiber ring,and some necessary modifications were made to remove the effect of the fiber birefringence. A theoretical analysis of optical path and locating method were also given in this paper,which demonstrated the feasibility of pipeline intrusion(or leakage) detecting and locating by this system.