The phase-sensitive opti cal time-domain reflectometer (-OTDR),as a new dis tr ibuted fiber sensor technology,has very significant practical value and economic value.A distributed optical fiber intrusion monitoring system based on coherent Rayleigh backscattering is built by using a kind of improved two-stage EDFA structure.Firstly,the optimal two -stage fiber length is obtained by theoretical simulation with OptiSystem software.Then,an improved method by div iding the signal into several segments is proposed to process the Rayleigh backscattered signal based on the f eature of coherent Rayleigh backscattered signal,which solves the problem that the power of demodulation si gnal is inhomogeneous. According to the randomness of scattering along the fiber,a discrete model of Ra yleigh backscattering is built and the Monte Carlo method is used to simulate Rayleigh backscattered signal.The experi mental results show that an approximate sensing distance of 35km with a spatial resolution up to 15m is reached.This system is potential to be used widely in safety monitoring of oil pipeline,airport,jails a nd so on.