Due to the problems of large rebar diameter, large volume, large number of distribution layers and complex installation, it is necessary to adopt a specific profile steel support system to assist the construction. Suzhou Zhongnan Center is taken as an example, according to the shape of the foundation pit, the arrangement of the reinforcement of the giant column of the shear wall, the arrangement of the elevator shaft, etc., the large-span profile steel support is reasonably set up to reduce the number of columns and ensure the convenience of reinforcement laying, handling and turning. At the same time, according to the deduction of construction conditions, the stress of the steel support at each stage is analysed, and the steel specifications and connection nodes are optimized. The final practice proves that the combination of high-strength bolt connection and welding connection can form a large-span assembled steel support for extra-thick foundation slab, which can greatly reduce the welding workload; At the same time, the overall hoisting after ground assembly can reduce the hoisting workload, speed up the construction progress of reinforcement, speed up the closing time of the bottom plate and ensure the overall safety of the foundation pit.