The apparatus and method for measuring displacement according to the present invention includes a first beam and a second beam. A first reflection structure reflects a first beam to the surface of an object under test; and a second reflection structure reflects a second beam to the surface of the object under test. The reflected first beam and the reflected second beam have an optical path difference. The object under test scatters a scattering beam of gathering the first and second beams. The scattering beam has an interference signal. A photodetector receives the interference signal of the scattering beam. Then an operational unit receives and computes the interference signal for producing a displacement value. By using the first and second reflection structures, the first and second beams split from an incident beam produce an optical path difference. Thereby, the structure of the apparatus for measuring displacement can be simplified. Besides, the displacement or the amplitude of vibration of the object under test can be measured effectively.