The supersonic nozzle is an import part of trisonic wind tunnels to obtain the flow field with designed Mach number, and the structure of nozzles has direct influence on the uniformity of flow field.Usually, for easy construction and convenience in the modification of nozzle contour, epoxy resin is adopted in the structural design of large scale nozzle such as a nozzle with exit size of 1,2m × 1.2m.However ,due to the expansion and contract caused by the heat and cold air flows during the operation of wind tunnel, there often appear some cracks on the edges and body of the nozzle made of epoxy resin;due to the large size of particle contained in the air flow, there often appear some nicks on the nozzle contour.The cracks and nicks will bring about shock waves in the flow field which has disadvantageous effect on the quality of flow field.In order to eliminate the shortcoming caused by the epoxy resin, the design of nozzle structure with aluminum and steel are considered.Through optimization of structural design of the nozzles made of aluminum and steel, the problems of high cost and difficult to construct were solved for the nozzle with steel structure, so a nozzle of steel structure is made with exit size of 1,2m × 1.2m and Mach number of 2.0.A great deal of experiments have been conducted on the nozzle, and experimental results show that the flow field of the nozzle is always good, and satisfies the requirements of GJBl179-91.In this paper, the optimization of structure design is described in details , some typical experimental results of flow fields of the nozzles with epoxy resin and steel structure are also presented.