A distributed optical fiber sensor system is constructed based on the principle of Sagnac interferometer with 3 ×3 coupler for long-distance pipeline security monitoring.By using this system,disturbance such as the action threat the security of pipeline can be identified and be located in real time.The system using ring-structure need not the phase modulation and demodulation because the 3×3 coupler can modulate the phase of optic signal,then the precision of signal processing is improved.The principle of the monitoring in ideal condition is put forward,the phase demodulation and the location of the possible damage action point are analyzed and formulated.The optic signal propagation and interference are simulated,interference signals in different frequencies are obtained.The results validate the feasibility of the system,indicate the system can detect the low frequency signal resulting from the damage action and locate the action point accurately over a long distance.