Abstract: In the original carbon-fixing device of multi-purpose furnace, the surface of the carbon-fixing sheet is prone to oxidation and discoloration, which affects the carbon-fixing results. By reforming the structure of carbonfixing sampling hole and installing nitrogen cooling device, the length of carbon-fixing rod is increased. When the fixed carbon sheet is taken out, nitrogen gas is introduced for protection and cooling, so that the qualified rate after the fixed carbon sheet is taken out reaches 100%. It reduces the cost of manual operation, improves the precision of carbon determination, and ensures the product quality.