The existing research on foundation pit mainly focuses on the deformation caused by soil excavation. It is often believed that the retaining wall deformation would commence after soil excavation. However, some field measurements show that the dewatering conducted before soil excavation might have already induced retaining wall deflection and surrounding ground deformation, which could be at centimeter level. Apparently, the monitoring data excluding deformations caused before excavation will underestimate the environmental effect of the construction work. In order to investigate the mechanism of foundation pit deformation caused by dewatering before excavation, a similarity model test is conducted to simulate the dewatering process. In the laboratory experiment, by means of mini dewatering wells, the seepage process around wells is reproduced and the seepage effect on retaining wall deformation is studied. The test results indicate that the depression cone outside the foundation pit enlarges continuously during the dewatering. Meanwhile, cantilever-type wall deflection and spandrel-type surface settlement are observed. In addition, the dewatering leads to an apparent decrease of the pore water pressure in front of the wall, which further induces decrease of the lateral total pressure. In response to the stress change, the retaining wall moves towards the pit in a cantilever type to achieve a new equilibrium state, and further induces the surrounding ground deformation as a result of the ground loss behind the wall.