It measures the vibration velocity of an object based on the Doppler principle. The Doppler principle means that if the wave source or the observer receiving the wave moves relative to the medium in which the wave propagates, the frequency measured by the observer depends not only on the frequency of vibration emitted by the wave source but also on the motion of the wave source or observer. The magnitude and direction of the velocity. The difference between the measured frequency and the frequency of the wave source is called the Doppler shift. When the vibration direction is consistent with the direction, the Dopp frequency shift is fd=v/λ, where v is the vibration velocity and λ is the wavelength. In the laser Doppler vibration velocity measuring instrument, due to the light round trip, fd = 2v/λ. This kind of vibrometer converts the vibration of the object into the corresponding Doppler frequency shift by the optical part during measurement, and converts the frequency shift into an electrical signal by the photodetector, and then sends it to the multi-channel after proper processing by the circuit part. The Doppler signal processor converts the Doppler-shifted signal into an electrical signal corresponding to the vibration velocity, which is finally recorded on a magnetic tape. This vibrometer uses a helium-neon laser with a wavelength of 6328 angstroms (┱), uses an acousto-optic modulator for optical frequency modulation, uses a quartz crystal oscillator and a power amplifier circuit as the driving source of the acousto-optic modulator, and uses a photomultiplier tube. For photoelectric detection, a frequency tracker is used to process the Doppler signal. Its advantages are that it is easy to use, does not require a fixed reference frame, does not affect the vibration of the object itself, and has a wide measurement frequency range, high precision and large dynamic range. The disadvantage is that the measurement process is greatly affected by other stray light.