Abstract: The design depth problem of excavation protection structures is one of controversial issues in the design of foundation pits. Taking the soil-nailing structure for the study, the strength reduction FEM is employed to solve this problem. The design depth of local ultra-deep excavation such as base beams and pile caps in the excavation is studied. Research shows that: the strength reduction FEM can be applied to the limit design of slopes and soil-nailing structures; different local ultra-deep excavation depths, sizes and distances are respectively treated in order that the soil nailing structures are truly designed reasonably, safely and reliably. In the design, the base beam ultra-deep excavation can not be considered, 1/2 of the ultra-deep excavation depth can be considered for the caps with 1~3 piles, and the ultra-deep excavation depth should be considered for the caps with many piles.