A novel detection of the frequency of the Fiber Bragg Grating vibration based on the relative intensity demodulating of an arrayed waveguide grating (AWG) is presented. The shift of the Bragg wavelength of the FBG sensors caused by the vibration can be demodulated by the relative intensity change of the corresponding pair of the AWG's two-adjacent-output-channels. Their photoelectric signal is acquired by the data acquisition card, and then a frequency analysis is carried out to get the frequency of the vibration. The experimental results are in line with the set frequencies of the vibration source. With this technique, errors caused by the light source fluctuation and micro-bend losses can be eliminated, and it will be more convenient to realize the frequency detection of the FBG distributed sensing system.