In monitoring intrusion incidents based on phase-sensitive Optical Time Domain Reflectrometer(φ-OTDR), the method compositing the past records of a single point and the data of neighboring points distinguishes intrusion from noise by the data change in a period and in neighboring area, and the system can locate the intrusion point where the optical intensity will change for the reason of interference.Based on this method, this paper is about a research on three parts, they are the threshold processing based on self-adaption mean value, analysis and simulation on frequency responses, and classifying the frequency information consists in the intrusion incidents.A group of experiments in 25 different frequencies confirm that the threshold has a good adaptability, the frequency responses agree with analysis and simulation, and the algorithm can classify the frequency information accurately.Therefore, this research will contribute to applying φ-OTDR to security system.