According to the dynamic population grating formation mechanism, a fiber-optic system with linear double-wave mixing structure is designed. In the system, light from a semiconductor distributed feedback (DFB) continuous wave(cw) laser operating at 1490 nm, as the incident light, is launched to a 3-m Er-doped optical fiber through a fiber optic circulator. The incident light is reflected from the vibrating mirror mounted on a piezoelectric actuator as reflected light. The dynamic population gratings are recorded along the longitudinal direction of the Er-doped optical fiber by spatial hole burning in standing-wave cavity formed by the two counter-propagating mutually coherent recording waves. The two-wave mixing (TWM) signal is observed as an intensity modulation wave in the output of the circulator when the reflected wave from vibrating mirror is phase modulated. Mechanical vibration induced by piezoelectric vibrating mirror can be detected through the designed two-wave mixing system. The results demonstrate that the two-wave mixing detection system displays a good dynamic response, with a frequency range of 50 Hz~10 kHz, and the frequencies of acquired vibration signals exactly agree with the driving signals from piezoelectric actuator.