Abstract: Micro-variation capacitance sensors are designed for use in guiding border-lines or confines. It makes use of the FFT(fast Fourier transform) arithmetic the phase differential coefficient to check and process signals based on the transmission line theory. High frequency characteristics of the coaxial-cable are used in detecting the signal, and low frequency characteristics are used in their location. Experimental results in a wall alarm system show that the sensor's relative error ratio in location is not more than 6.0 percent, the error alarm ratio is not more than 0.3 percent, and the miss alarm ratio is not more than 0.2 percent.