The invention discloses a plastic-lined steel pipeline external plastic-coating method. A material conveying device and a lower bracket are mounted on an original rotational molding machine; the material conveying device is composed of a material conveying pipe and material spraying pipes mounted on the material conveying pipe, and the material conveying device is used for spraying fusible plastics onto a steel substrate to form a plastic-coated layer; the material conveying device and the lower bracket are fixedly provided with a scraping plate device, and the scraping plate device is composed of a spiral scraping plate, guide rods, positioning columns and pressure springs; when the heated and discharged steel substrate with high temperature is placed on a rotational molding machine, the material conveying device and the scraping plate device are reset and fixed onto the rotational molding machine, after the rotational molding machine is started, the steel substrate can rotate in the spiral scraping plate, the material conveying device is timely started, the material spraying pipes spray a fusible plastic powder onto the steel substrate, the spiral scraping plate on the scraping plate device can make the fusible plastics scraped uniformly, and a thin-film protective layer is formed on the surface of the steel substrate; and when the external plastic-coated layer reaches the requirement, the material conveying device is closed, and material spaying on the plastic-coated layer is stopped.