Calculation model of prestressed concrete top beams of foundation pit is proposed.The basic principle of prestressed concrete top beams is to use the equivalent load effect of prestressed concrete.The equivalent load provided by prestress can balance part of earth pressure.The simulation results show that

（1） the prestressed concrete top beams can increase the top stiffness of the support system and improve its stress condition.Furthermore

the construction cost can be reduced.（2） the calculation model of the prestressed top beams is simple.The prestressed top beams have the same effect as inside support and tensioned cables.The construction is more convenient and the engineering quality is more easier to control

（3） the tensile function of prestressed reinforced rebar can be fully exerted

the displacement of retaining structure on the top can be reduced

and the supporting depth of cantilever retaining structure can be increased

（4） it is more economical and environmentally friendly

the prestressed reinforced rebars in the beam can be recycled and reused.It is proved that the prestressed top beams are wothy to be applied in the foundation pit supporting structures.