A pair of fiber Bragg gratings with the wavelengths of 1562.11 nm and 1562.71 nm, respectively, are used as the end reflectors of a constructed linear cavity fiber laser. They are epoxied onto the upper and lower surfaces of an employed uniform density and thickness isosceles triangle cantilever beam axially. As a strain agent, the beam makes the wavelength shifts of both gratings be changeable, and therefore lead the laser producing pulses. The real time measurements of the frequency of the mechanical vibration applied on the free end of the beam are demonstrated by observing the pulses. Within the range of 0～6.1 mm, the amplitude of the vibration can be estimated by observing the wavelength shift difference between two gratings. The measured results have been demonstrated to be in good agreement with the actual values under the frequency of 100 Hz.