Composite material flap doors are made of specially developed load-bearing materials. At the beginning of developing this material, the quality ratio, corrosion resistance, strength, and price factors were considered. The flap doors produced from this have better corrosion resistance and lower quality ratio compared to traditional cast iron flap doors and steel flap doors. The small opening pressure only requires 0.0003MPa, far superior to traditional flap doors. Compared with stainless steel flap doors, its low cost is a decisive advantage. Composite material flap doors are an ideal product to replace traditional flap doors. The door seat adopts a double flange structure, with the lower flange serving to connect and fix the door, and the upper flange serving to seal, stop water, and drain water. The fixed flange of the circular flap door adopts the standard and is compatible with the steel flange sold in the market, while the fixed flange of the rectangular flap door is a non-standard flange.