Micro-deformation are important in engineering to determine the health state of a target or to warn of natural disasters. Traditional deformation monitoring methods include displacement meter, accelerometer, GPS, 3D laser scanning, etc. However, they have certain disadvantages, such as installation difficulties, low working efficiency and weather sensitivity. Therefore, a solid target micro deformation Meter (TMDM) is designed. The system combines the frequency modulation continuous wave technology (FMCW) and interferometry, which is easy to operate, portable, and can operate all day long. Firstly, the system is tested in laboratory, and the accuracy of deformation measurement is verified to be up to sub-millimeter. Then, the system is used to carry out a micro-deformation monitoring experiment on a 32m double-line simply supported box girder of Haian Bridge. During the experiment, four typical monitoring points are selected. Under the experimental condition of single line loading and the same loading speeds of the girder, the system accurately measured the displacement of 1/4 span and 1/2 span of the beam and the displacement difference of 1/2 span of the uplink and downlink. The natural frequency and forced frequency of the bridge are obtained by further analysis of the measured time-history curve. The measured results are consistent with the theoretical analysis, proving the effectiveness of the TMDM.