In this study, we design a low-cost and long-perimeter security system using the weak grating array technology and phase-sensitive optical time domain reflectometer. The developed system uses the grating positions and the time-division multiplexing technique to precisely locate the external vibration points. The signal-to-noise ratio of the demodulated vibration signal is approximately 30 dB according to the principle of interference between the reflected light of the grating and the back Rayleigh scattering light of the two sides of the fiber. Herein, we use a fiber optic cable with a 5-m grating spacing to demodulate the vibration having a frequency of 10--90 Hz, and the demodulated results prove the excellent frequency response characteristics of the system. Meanwhile, the alarm algorithm of the proposed perimeter security alarm system is verified. The test results prove that the false alarm rate of the system is less than 10% and that the missing alarm rate is 0. The developed system has a maximum detection length of 50 km, which is considerably larger than those of the existing optical fiber perimeter products, and the demodulation method is simple and cheap; therefore, it has a broad application potential.