Cut-off wall is widely used to control the seepage field for building’s foundation pit, but the efficiency of cut-off wall is not known very well. In the background of the common foundation of double layers in Wuhan of China, a model was set up for the seepage field around a typical building foundation pit. Through numerical simulation, the law of seepage field control for cut-off wall was revealed. Then the application of cut-off wall was discussed on the basis of the study results and engineering practices. It is pointed out that cut-off wall of closure type could control seepage field efficiently under the conditions that the cut-off wall is prevented from any defect and is extended into reliable stratum with low permeability. The seepage control efficiency can be promoted to limited extent for the cut-off wall of suspension type with higher penetration rate which should be set reasonably with the cost, duration, water discharge and safety being taken into account. An efficient seepage field control system must be formed by coordinating cut-off wall and drainage measures. The seepage field control system for a building foundation pit should be designed in consideration of adjacent buildings, flood control projects and other facilities in addition to geological and hydrological conditions. Then we can achieve the safety of foundation pit and control its adverse effect on environment. These conclusions are referential for the foundation pit of public works including metro engineering.