According to the first harmonic cavity-length locking technology, a novel kind of micro-displacement sensor, which combines F-P interferometer and intensity demodulation method, has been developed. The initial length of the F-P cavity has been actively scanned and dynamically locked. And then the change of the F-P cavity-length induced by the motion of the target is coded on the variation of the optical output power of the cavity. As a consequence, a fast and direct micro-displacement sensing scheme through the intensity demodulation is available.The theoretical model of the displacement sensor and the technical scheme for actively locking the initial length of the F-P cavity through the first harmonic have been demonstrated in detail, respectively. Micro-displacement experiments provided by a commercial high-precision PZT with different vibrating parameters have been performed, and the experimental results agree well with the motion of the PZT within peak-to-peak amplitude of λ/4 and frequency is no more than 400 Hz. The frequency error is less than 0.5 Hz, and the total measurement accuracy is less than 1 nm.