Abstract: The synthetic experiment facility is a high pressure and high temperature piping system. The de-sign pressure is 17. 5 MPa and temperature 350℃. Because of the needs of simulating experiment, thedesign of piping system is restricted. Especially, because water temperature in the downcomer tubes ofsteam generator is 20℃ at the start--up condition, the safety consideration during operating is impor-tant. The thermal compensation of the piping system becomes difficult. According to the principle ofSDGJ6--78, by means of making anchor a displacement, raising water operating temperature in thedowncomer and designing a reasonable supporting system etc, the piping system operation is safety atgiven conditions.