Abstract: A method of measuring small vibration with an amplitude lower than half that of the wavelength of laser is developed. The characteristic of self mixing semiconductor laser, which can predigest the light path structure of the interference system, is introduced. The principle of laser Doppler vibration measurement is introduced. The pertinence of measuring small vibrations with the frequency analysis method when the wave number σ<1 is analyzed, and the small ultrasonic vibration with only 90 nm amplitude is measured through the method. Experiments show that the limit of the method will be reached when the maximum Doppler frequency equals to the vibration frequency.