The flue gas heat tracing tube is connected to the sampling probe and the pretreatment system. It is composed of two sets of corrosion-resistant high-performance tetrafluoroethylene pipes supplemented by high-temperature constant-power electric heating belts and compensation cables to form an inner core, plus an insulation layer of imported raw materials, and finally covered with polyethylene (PE) The protective jacket is compounded. The temperature in the sampling tube is controlled at about 120°C, so that the water content in the flue gas exists in a vapor state, preventing water condensation and SO2 from generating acid.