In this thesis, an intrusion detection system based on phase-sensitive optical time domain reflectrometry is studied, with a lab-made ring resonant cavity laser used as a light sourse. This laser exhibits a narrow linewidth of about 1KHz(FWHM), and its light is split into two parts,through a 10:90 fiber coupler, with the smaller part being a local oscillator light and the larger part the signal light injected into an AOM modulator to form a pulse train. The pulse train is launched into a test fiber of 4.4km in length.The Rayleigh backscattered light and the local oscillator light are then fed into a 50:50 fiber coupler and then balanced detectors.By a subtraction and mixer operation followed by low pass filtering, a detection signal train is generated. By using a moving average algorithm, we observe simultaneous intrusion points clearly enough with 1000 data windows processed in the algorithm.