Vibration signal frequency is measured by monitoring the single wavelength power of fiber Fabry-Pérot (F-P) cavity and applying fast Fourier transform (FFT) to the time domain signal. An instantaneous model is established and the minute response delay between transmission and reflection is shown. It is proved that the multi-reflection of F-P cavity has little influence on the measurement of vibration with a frequency generally not higher than some dozen kilohertz. The waveform will be distorted when the vibration directly applied on the F-P cavity is intense and out of the demodulation monotropic range. Desensiblization is implemented by increasing the distance between the vibration source and the F-P cavity to control the monodromy of demodulation and more accurate demodulation results are obtained when the distance is increased to 8 cm.