We use highly coherent and very low-noise laser as the light source to design fiber Michelson interferometers with an optical path difference of 60 m used as the front-end transducers. Binary rectangular pulse is used to modulate the phases of the light source, and the phase signals are restored with an orthogonal demodulation algorithm. A highly efficient interrogator controller based on field programmable gate array(FPGA) is used to calculate phase signals in real time. Real time and accuracy of invasion alarm signals are improved, based on the time-domain level crossing (LC) algorithm. Finally, a four-zone security system is set up, which has advantages of simple interrogator controller, low cost and real-time handling capability. The experimental results show that the system can alert all kinds of intrusions in real time and suppress the false alarm caused by the environmental noises, such as winds and rains, under the chosen proper threshold. The system has worked for 1 month without alarm failure and the false alarm rate is less than 1%.