The air duct has good rigidity, insulation effect, corrosion resistance, and weather resistance; The surface is smooth, aesthetically pleasing, dust-free and non sticky, especially suitable for clean engineering exposed air ducts. This air duct has excellent energy-saving and economic performance. The air duct itself has a very small air leakage, a good insulation layer, and eliminates "cold bridge" flanges. The three major problems are solved simultaneously, greatly reducing the heat loss of the pipeline.