The invention discloses a plastic coating device for a large-diameter steel pipe. The plastic coating device for the large-diameter steel pipe comprises a base frame. The base frame is provided with steel pipe rotating devices. A guide rail is arranged and parallel to the rotation axis of the steel pipe borne by the steel pipe rotating devices. Walking frames driven by a power device are arranged on the guide rail. The walking frame is provided with external plastic coating device bodies and internal plastic coating device bodies. Each internal plastic coating device body is located below the corresponding external plastic coating device body. Each external plastic coating device body comprises a powder material groove. The powder material grooves are fixed to the walking frames. Valve plates driven by the power device are arranged at feeding openings of the powder material grooves. Each internal plastic coating device body comprises a material overturning shaft. The material overturning shafts are rotationally installed on the walking frames. One end of each material overturning shaft is connected with an overturning drive mechanism. The other ends of the material overturning shafts are fixedly connected with material overturning hoppers capable of axially extending into the steel pipe. The plastic coating device can conduct internal plastic coating and external plastic coating on the large-diameter steel pipe at the same time and is high in coating speed, the coatings on the steel pipe are uniform, and the powder recovery rate is high.