Focusing on the foundation pit of an ultra -deep working well in Foshan City, numerical simulation methods are used to analyze the influence of different foundation pit sizes on the horizontal displacement, bending moment, surface settlement behind the wall, and pit bottom uplift of the diaphragm wall, and the reliability of numerical analysis is verified by on-site monitoring results. The influence of the size effect on the arch effect characteristics of the circular foundation pit is further studied. It is found that that the horizontal displacement, bending moment, surface settlement behind the wall, and pit bottom uplift of the diaphragm wall show an overall trend of increasing with the increase of the size of the foundation pit. When the diameter of the foundation pit is less than 65.9 m, it has a greater impact on various parameters. And when the diameter of the foundation pit is about 0.6~0.9 times of the excavation depth of the foundation pit, the circumferential stress of the retaining structure is greater than the radial stress, which has a good "arch effect"; when the diameter of the foundation pit exceeds this range, the hoop stress is smaller than the radial stress, and the "arch effect" is weakened. Through the study of the size of an ultra-deep circular foundation pit, it is expected to provide reference for related projects.