Abstract: Canton Tower South Square is a large underground space building with two floors underground and one floor above the ground. The single floor area is about 43000 square meters. This kind of large basement adopts top-down construction method, which has its particularity. This paper discusses the whole process of structural design of top-down construction method, analyzes the support structure selection, support column selection, foundation selection and excavation scheme of top-down construction method, and puts forward feasible scheme for key problems such as structural design principle, beam column joint design and construction requirements. At present, the project has completed the landscape engineering of the ground floor and the platform floor above the ground, and the underground excavation and structural construction are in progress. This paper summarizes the structural design method of top-down construction method for this kind of large underground space, which can provide reference for the structural design of similar projects.