Abstract: The size of the foundation pit affects the deformation characteristics of its supporting structure. The effect of long top beams in a long foundation pit is not quite clear. In order to clarify the force state,deformation form and the influence on other supporting structures,we studied the load conditions and restraint conditions of long and short top beams on the basis of previous studies,and obtained their main deformation characteristics. With the help of numerical simulation,we verified the long top beam's deformation form,and assessed effect of different level of integrity and the setting of prestress on the anchor rod. Then we proposed a new spliced top beam,based on the advantages and disadvantages of traditional top beam. Then we assessed its supporting effect and restrict ability on damage expansion by numerical methods. Study shows that the long top beams have more complete "wave characteristics" than the short top beams,which may deteriorate the stability conditions of local piles and hinder the function of prestressed anchors. The proposed new top beams are hinged and then rigidly connected,which weakens the "wave characteristics". It retains the function of the traditional top beam to strengthen the support structure and reduces the deformation and improves the effectiveness of bolt support. It turns out to be an optimized design.