Abstract: Using the finite difference method and the limit equilibrium method, the failure modes of excavations and the effect of foundation piles in excavations on excavation stability are investigated. The following conclusions are drawn: (1) The foundation piles can improve the safety factor of stability to certain degree. (2) Instability sliding surface of excavations with single level of horizontal struts is close to the circular slip surface rotating about the supporting point of the strut. (3) When instability of excavations occurs, the foundation piles tend to be subjected to bending failure rather than shear failure. While taking the contribution of foundation piles into consideration, the smaller value between the equivalent shear strength and real shear strength should be used.