The steel pipe diagonal brace is often used to rescue and protect the sidewall of the excavation when the ground deformation reaches the alert threshold. However, its bearing capacity depends on the stability of the ground close to the diagonal brace foot. In the proposed study, a series physical model tests were conducted to study the stability of the diagonal brace foot. Experimental loading tests and numerical analysis were carried out on the diagonal brace foot with different embedding depths, the load-displacement curves and the deformation characteristics of the ground soil were obtained. Based on this, the bearing mechanism and failure modes of the diagonal brace foot were studied. When the embedding depth of the brace foot is shallow, the ground close to the brace foot is trend to be the passive failure state; when the depth is larger than a certain value, the bearing capacity of the diagonal brace foot can then be fully activated. The results of the experimental and numerical studies suggest that in engineering applications, it is necessary to ensure that the brace foot has an embedding depth of larger than 2 times the pipe diameter.