The invention relates to the technical field of 
welding, in particular to a compound 
pipe longitudinal joint automatic 
welding device which mainly comprises a double 
welding gun movement mechanism, a 
control system, welding power supplies and a welding fixture. The double welding gun movement mechanism is composed of a movable platform, a left welding gun, a right welding gun and a cross sliding frame, wherein the two welding guns are provided with wire feeding mechanisms and welding power supplies respectively. The 
control system is located on the left side of the welding fixture, and the two welding power supplies are connected with the 
control system. The welding fixture comprises a welding front-rear symmetric supporting device, a lower supporting device, a supporting mandrel, a pneumatic 
key pressing device and a precision movement sliding rail. According to the welding device, a double 
metal compound 
pipe longitudinal joint with a 
diameter of 200-500 mm, a length of 1800 mm and a thickness of 4-7 mm can be clamped quickly and reliably, the control 
system can finish welding device control, welding track planning, welding parameter setting and 
welding process monitoring, and the double 
metal compound 
pipe longitudinal joint is automatically welded.