Abstract: The method of plane design is adopted by the design of foundation pit support. As a three-dimensional space system with long, wide and depth, the plane design of foundation pit cannot meet the characteristics of three-dimensional space structure of a foundation pit. Based on the spatial effect of a foundation pit, the calculation method of active soil pressure and passive soil pressure under the influence of horizontal displacement of slope roof is proposed. With reference to the current specifications, the embedded stability safety factor under the influence of space effects is calculated, and the minimum embedded safety factor (the most dangerous state) is used as an optimization criterion to propose an optimal design scheme for a foundation pit, a cantilever supporting structure. The study results show that: the active soil pressure and the passive soil pressure are affected by the space effect simultaneously during excavation, but the active soil pressure is more affected by the spatial effect than the passive soil pressure. The embedded stability safety factor increases as a function of power as the distance from the pit angle decreases.