The invention discloses a large-diameter flame retardant pipe which comprises an inner steel pipe layer, a PVC (polyvinyl chloride) composite flame retardant layer, a nanometer ceramic layer, a bufferlayer and an outer steel pipe layer which are arranged sequentially from inside to outside, wherein the outer surface of the outer steel pipe layer is coated with an anti-corrosion coating. Structural strength of the large-diameter flame retardant pipe can be effectively guaranteed through the inner steel pipe layer and the outer steel pipe layer, flame retardant performance of the large-diameterflame retardant pipe can be enhanced through the PVC composite flame retardant layer and the nanometer ceramic layer which are located between the inner steel pipe layer and the outer steel pipe layer, buffer action can be achieved through the buffer layer even the outer steel pipe layer is deformed when the large-diameter flame retardant pipe is impacted by outer force, damage to the nanometer ceramic layer is avoided, and high impact resistance is achieved; therefore, compared with the prior art, the large-diameter flame retardant pipe has the advantages of good flame retardant performance,high structural strength and high impact resistance.