The deformation of the retaining structure and surrounding soil during the excavation of a deep and large triangular foundation pit in Shanghai soft soil was analyzed using finite element software. Meanwhile, the influence of the side length and the corner location of the triangular foundation pit on its deformation were studied based on its special triangular structure. The results show that the numerical model established in this paper can very well predict the deformation of the retaining structure and the surrounding soil caused by the excavation. The lateral displacement of diaphragm wall presented a “bulging” mode, and a surface settlement groove occurred behind the wall at a certain distance. Both the lateral displacement and the surface settlement increased with the continuous excavation, and showed significant spatial characteristics. The deformation reached the maximum at the middle point of the long side of the triangular pit, whereas the deformation was relatively small near the corners. The smaller the angle of the corner and the closer the measuring point to the corner point was, the smaller the deformation was. This study can provide some practical value for the design and optimization of the foundation pit in soft soil.