Abstract: The vibration characteristic of loudspeakers is investigated experimentally, based on the self-mixing interference effect of semiconductor laser diode. As a target, a reflector with reflectivity of 95% is mounted on the loudspeaker, which is driven by a sine signal at 50Hz frequency, and a self-mixing interference signal is obtained with an extremely simple optical setup. Then the vibration amplitude of loudspeaker is figured out through the interference fringe counting method. The results show that the vibration amplitude, which responses to the driving voltage, exhibits good linearity in corresponding range. It is a fast, low-cost and easy to perform method that does not require special driving device or other optical apparatus. This experiment illustrates the effectiveness of the self-mixing technique in the field of vibration measurements.