The acousto-optic modulator(AOM) is an important component of fiber-optic distributed vibration sensing system based on Φ-OTDR which can modulate the CW light to pulsed light. The frequency of AOM directly affects the dynamic performance of the system. Based on the sensing mechanism of the system, the influence of modulation frequency on the frequency response was analyzed theoretically. The results show that the utmost frequency of the vibration signal regenerated by the system is limited by the modulation frequency. The range of frequency response extends with the increase of the modulation frequency, which means the limitation of frequency response increases and the distortion of the vibration signal decreases when a laser source with constant optical power is utilized. On the contrary, the high frequency vibration signal will be severely distorted when the modulation frequency is insufficient.