Research conclusions: ( 1) The ground surface settlement during the underground structure being constructed may be larger greatly than that during the foundation pit being excavated when the underground structure is built in the soft site． Accordingly，the deformation of the foundation pit during the underground structure being constructed must be intensified． ( 2) The horizontal deformation of the support wall is very sensitive to the excavation thickness and the properties of soil layers every time． It is obviously that the cumulate horizontal displacement of the support wall becomes increasing faster with the excavated depth of foundation pit increasing． ( 3) When the excavated depth of foundation pit and the thickness of soil layers laying under the bottom of foundation pit change obviously，the differential settlement of upright column must be considered carefully． ( 4) The space － time variation of the foundation deformation is obviously when the soft soil layers existing around the foundation pit． The excavation size of soils must be designed carefully．