Powder spraying chamber: the workpiece enters the spray gun position of the powder spraying chamber through the conveying chain to prepare for spraying. The electrostatic generator releases high-voltage static electricity (negative pole) to the space in the direction of the workpiece through the electrode needle at the gun mouth. The high-voltage static electricity ionizes the mixture of powder and compressed air ejected from the gun mouth and the air around the electrode (with negative charge). The steel pipe is connected to the ground (grounding electrode) by means of transportation through the hanger, so that an electric field occupying powder is formed between the spray gun and the workpiece and reaches the workpiece surface under the double push of electric field force and compressed air pressure, and a uniform coating is formed on the surface of the steel pipe by electrostatic attraction.