Abstract: Confocal Fabry-Perot Interferometer (CFPI) is the demdulation device for surfce ultrasonic vibration detection. Its sensitivity and stability play very driportant roles in whole system. In this paper, a new detection system is introduced. By using both conjugate rays, the system sensitivity is doubled. Moreover, a single chip microcomputer (MCS-51) feedback control loop is applied to stabilize the working point at optimum position. The system has performed in detecting piezoelectric ultrasonic on surface of an aluminum sample.