Composite air duct is composed of two pipes with different inner and outer materials. The inner layer is aluminum alloy air duct, and the outer layer is steel air duct. Glass fiber insulation material is filled between the two to form a composite pipeline system. It has the advantages of multiple materials and can meet the needs of ventilation, smoke exhaust, and air supply systems inside buildings. At the same time, it can effectively resist the spread of fires and improve the fire safety factor of buildings.