Combined with an actual case, the software package of FLAC3D finite difference method is used to conduct the numerical simulation of the construction of the building foundation pit adjacent to an existing tunnel and to obtain the upward vertical displacement of tunnel and the horizontal displacement against the foundation pit caused by the excavation and unloading of foundation pit. The maximum displacement occurs at the tunnel position opposite to the excavation of foundation pit. And the smaller the horizontal distance from the foundation pit, the larger the displacement. After the excavation of the foundation pit, the maximum vertical displacement value of the tunnel is 2.8 mm (vertical upward), and the maximum horizontal displacement is -0.4 mm (against the foundation pit direction), which has a small impact and is within the safe and controllable range.