At present, most studies on the spatial effect of open-cut foundation pits focus only on the influence of either internal corners or external corners on the supporting structure, while the coupling effect of internal corners or external corners on the deformation characteristics of the retaining structure and surrounding soil has not been thoroughly investigated. Based on the Gongming Square open-cut foundation pit project of Shenzhen Metro Line 13, this study constructed a three-dimensional numerical model. The influence of the coupling action between the internal and external corners during excavation on the deformation of the retaining structure was analyzed. The deformation pattern of lateral displacement of the diaphragm wall under different relative positions of the internal and external corners was obtained, and a calculation formula for the influence coefficients of the internal and external corners was proposed. The results show that an increase in the wall length of the external corner parallel to the long side of the pit only affects the lateral displacement of the diaphragm wall on the pit side where the external corner is located. Increasing the wall length of the external corner is beneficial to control the horizontal displacement along the long side but is unfavorable for controlling displacement along the short side. Surface settlement at 1 m behind the diaphragm wall increases with the wall length of the external corner (parallel to the long side), and the point of maximum displacement shifts from the central part of the pit toward the location of the external corner.