Only by ensuring the quality of the joint assembly of the pipes can the quality of the welding be guaranteed. Uneven thickness of the pipe wall and roundness deformation of the pipe end will make it difficult to assemble the pipe sections. Pipes with a diameter of 300 mm or more use a pneumatic or hydraulic internal coupling device, which can automatically round the pipe end and adjust the gap between the couplings. A welding device is installed on the inner mouthpiece with a pipe diameter of more than 1000 mm to perform welding operations inside the pipe to improve the efficiency and quality of assembly and welding.