The instability failure of many deep excavations supported by diaphragm walls (retaining piles) and horizontal struts is caused by the local failure of struts and the following large area chain effect. The lack of redundancy of struts is an important reason for the overall failure of supporting structures. In this paper, based on an actual excavation project, the numerical calculation model is established by Flac^3D5.0, and the reliability of the supporting structure is analyzed based on the redundancy theory. The main conclusions are as follows: the redundancy of single support is large, and strut (6) (close to the middle of the excavation) is the most important. The redundancy is reduced due to continuous failure, and the redundancy is only 3.50 when strut (1)–(7) are all failed (half of the struts). The second row of the struts has the smallest redundancy, while the third row has the biggest redundancy.