In order to effectively evaluate the collapse resistance of fabricated steel support system consisting of prestressed fish-bellied beams in excavation, the redundancy of prestressed support system was studied by using the demolition member method. Based on the verified three-dimensional finite element model of steel support system, the whole process simulation of deep foundation pit construction is realized, and the variation rules of the axial force of the prestressed reinforcement and the displacement of the piles in different excavation stages are obtained. The results show that in the construction process, more attention should be paid to the monitoring of the axial force of the prestressing tendon and the support, and the deformation of the foundation pit should be controlled by timely tensioning the prestressing tendon and adjusting the pre-applied axial force of the support. The member redundancy of the two-layer support is smaller than that of the first layer support; In the transverse comparison of the members, the redundancy of the opposite support is the smallest, that of the corner brace is the second, and that of the prestressing tendon is the largest. That is to say, the failure of the opposite support has the worst resistance to the progressive collapse of the foundation pit.