During the construction of highway, railway and subway tunnel projects, the sections where the frame structure has not been re-strengthened are subject to a variety of internal and external stresses and are prone to deformation. when the deformation exceeds a certain threshold, there will be landslides, settlement and other dangerous situations. In order to solve this problem, a distributed millimeter wave radar deformation monitoring system is exploited and a high precision ranging algorithm is proposed to realize three-dimensional micro-deformation measurement of the target. Field experiments show that the system together with the deformation monitoring algorithm realizes the real-time deformation monitoring of tunnel cross-section structure during construction, and its accuracy and stability meet the requirements of tunnel environment. It provides a great safety guarantee for tunnel construction.