Insert a thermocouple into the muffle furnace with its thermal node located at the center of the furnace as a reference, and insert another thermocouple or tree thermocouple into the furnace as a measuring device. Heat the muffle furnace to the operating temperature (900 ℃ or 815 ℃), and stabilize the furnace temperature at this temperature using a temperature controller according to the reference thermocouple instructions. Move the measuring thermocouple along the front, back, left, right, and up and down directions. The distance of movement depends on the temperature gradient of the muffle furnace. The distance can be larger when the gradient is small, and smaller when the gradient is large. Generally, move 1-2 cm each time. After each movement, maintain a constant temperature for 3-5 minutes at the predetermined temperature, Read the temperature indicated by the millivolt measurement thermocouple, and finally locate the constant temperature zone of the muffle furnace based on the temperature at each measurement point.