Vacuum furnace is a kind of graphite heater used to melt polycrystalline materials such as polysilicon in an inert gas (mainly nitrogen and helium). Equipment for growing dislocation free single crystals by Czochralski method. The vacuum furnace has strict temperature requirements. For example, if the processing temperature of the furnace body is not effectively controlled, the furnace body will be deformed due to excessive temperature. It is bound to affect the change of equipment size and cause the appearance of unqualified products; At the same time, the furnace body materials work at high temperature for a long time, and uneven cooling and heating will reduce the service life of the furnace body and cause losses.