The horizontal displacement of the diaphragm wall during the excavation of the foundation pit is an important indicator of its stability. The internal force distribution and structural deformation of diaphragm walls during the excavation of foundation pits is investigated in this paper. Taking a foundation pit project in Beijing as an example, the deformation of the diaphragm wall during the excavation process is monitored using ultra-weak fiber optic grating technology and numerical simulated through the Comsol Multiphysics finite element software. The results show that ultra-weak fiber grating technology can accurately monitor minute deformations of diaphragm walls, revealing the piecewise variation of strain along depth. The location of the anchor bolts has a significant impact on the vertical distribution of deformation of the diaphragm wall, and will change the shape of the displacement curve. In this case, the strain of the second and third anchor bolts is approximately zero, which corresponds to changes in the concavity and convexity of the displacement curve.