Abstract: Thermal insulation device should be set in the steam generator to improve the efficiency of the heat exchange and compact the cold and hot helium. The thermal insulation fiber was laid on the steel cylinder around the heat exchange tubes, and packed firmly by steel plates. The plate was fixed in the middle and four corners by the bolts filled with thermal insulation fiber. The plates and bolts compose the supporting structure of thermal insulation layer. The stress distribution was calculated via finite element method (FEM) under the load of thermal insulation fiber pressure, loss of steam generator pressure and seism. The results show that the supporting structure of thermal insulation layer can keep its reliability and integrality under the definite loads.