Abstract: In order to locate the long distance perimeter intrusion, an optical fiber intrusion detection system was promoted based on Fox-Smith interferometer with 33 coupler as the main component. Theoretical analysis and experimental verification were carried out on the system and its location method. When the impact force acts on the sensor fiber, the optical wave is modulated by the phase. There is phase difference because of the modulation time difference between both the beams. A series of trap points are gained after the fast Fourier transform of the demodulated phase signal. The results showed the average location error was 83m when the length of sensing fiber was 34.793km.The system is feasible for long intrusion location and helpful to intrusion detection.