A distributed optical fiber sensor system based on the principle of Sa gnac optic interferometer with 3×3coupler is developed for long-distance pipeline security monitoring.By using this system,t he disturbance which may threat the security of pipeline can be identified and located in real time.The system using ring-structure Sag nac interferometer does not need the phase modulation and demodulation,because the 3×3coupler can modulate the phase of optic signal,and th en improve the signal disposal precision.The principle of the monitoring in ideal condition,the phase demodulation and locat ion of the possible damage action point are analyzed and formulated.Based on the theory,the ring structure Sagnac interferom eter real-ti me monitoring system and the disturbance signal resource are set,the data collection software is designed,the interfere nce signals demodulated by different frequencies of disturbance signals are collected,and then the data are demodulated and calculat ed to analyze the disturbance position.The results validate that the largest average absolute error of location is 1.23% when the disturbance frequency is between 100Hz and 500Hz.The sy stem and indicate that the system can detect the low fr equency signal resulting from the damage action and locate the action point accurately over a long distance.