A　distributed　optical　fiber　sensor　system　based　on　the　principle　of　Sagnac　optic　interferometer　with　3×3　coupler　is　developed　for　long-distance　pipeline　security　monitoring.　By　using　this　system,　the　disturbance　which　may　threat　the　security　of　pipeline　can　be　identified　and　located　in　real　time.　The　system　using　ring-structure　Sagnac　interferometer　does　not　need　the　phase　modulation　and　demodulation,　because　the　3×3　coupler　can　modulate　the　phase　of　optic　signal,　and　then　improve　the　signal　disposal　precision.　The　principle　of　the　monitoring　in　ideal　condition,　the　phase　demodulation　and　location　of　the　possible　damage　action　point　are　analyzed　and　formulated.　Based　on　the　theory,　the　ring　structure　Sagnac　interferometer　real-time　monitoring　system　and　the　disturbance　signal　resource　are　set,　the　data　collection　software　is　designed,　the　interference　signals　demodulated　by　different　frequencies　of　disturbance　signals　are　collected,　and　then　the　data　are　demodulated　and　calculated　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　100　Hz　and　500　Hz.　The　system　and　indicate　that　the　system　can　detect　the　low　frequency　signal　resulting　from　the　damage　action　and　locate　the　action　point　accurately　over　a　long　distance.　©,　2015,　Board　of　Optronics　Lasers.　All　right　reserved.