Abstract: The pipeline internal circumferential automatic welding machine (PIW machine) has been mainly used for pipe alignment and welding root pass in course of pipeline construction. Compared with the other root pass equipment and technologies, PIW machine has the higher efficiency and better root quality. This paper researches the key technologies such as the entire design of PIW machine with multi-torches, digital control system, arc-control for small size wire and the internal welding process. The site application indicates PIW machine with multi-torches has the advantages of reasonable structure and advanced control system, its welding speed is 90s per joint. Its reliability and the filed adaptability to environment meet the site welding construction request completely.