A device and method for measuring amplitude of a scanning mirror are disclosed. The device includes a light source (20) for outputting an optical signal; a diaphragm (21) for modifying size and shape of a light spot of the optical signal output by the light source (20); a scanning mirror retainer for placing a scanning mirror (22) to be measured, the scanning mirror, after being retained, being able to periodically reflect the optical signal; a photoelectric sensor (23) including three or more sensing elements and configured to detect and collect the optical signal reflected by the scanning mirror (22); and a signal acquisition and processing unit (24) for processing a signal collected by the photoelectric sensor (23) to derive an amplitude of the scanning mirror (22). Therefore, it is possible to characterize the scanning mirror (22) before the scanning mirror (22) is used, determining performance of the scanning mirror (22).