The invention relates to a novel rectangular glass reinforced plastic flue comprising a sandwich type flue frame structure and an inner brace rod structure. The sandwich type flue frame structure comprises inner and outer glass reinforced plastic surface layers. Sandwich type circumferential-direction stiffening ribs shaped like the Chinese character ''jing'' are arranged between the two reinforced plastic surface layers through an adhesive layer. The inner brace rod structure is composed of inner brace rods forming a net structure shaped like the Chinese character ''jing'' through assembly welding. Circular tubes are welded on the sandwich type circumferential-direction stiffening ribs shaped like the Chinese character ''jing'', penetrates through the inner glass reinforced plastic surface layer and is connected with the ends of the inner brace rods in a welding mode. The novel rectangular glass reinforced plastic flue has the beneficial effects that a composite material structure composed of the corrosion resistant sandwich type flue frame structure and the inner brace rod structure is adopted; through the structure, the disadvantages of low elastic modulus and poor rigidity of glass reinforced plastics can be effectively overcome; meanwhile the overall strength and rigidity of the glass reinforced plastic flue can be strengthened; under the same bearing capacity, the weight of the structure can be reduced greatly; and the inner brace rods can disperse the borne force of the rectangular flue, the situation that the ends of the inner brace rods damage a glass reinforced plastic wall plate due to expanding with heat is prevented.