Abstract: This paper presents an all-position welding controlling system for thick-wall piping with narrow gap,which adopts two grade decentralized computer control mode.In this system,there are one 486 notebook computer used to perform the input/output of the data and management and seven single chip microcomputers (80c552) as local control grade units to complete real-time control, which include the swing control and the horizontal adjustment of tungsten electrode, the adjustment of arc length, the travelling speed control of torch,the feeding control of welding wire and the remote and auxiliary control etc. Standard RS-232 DEC is adopted for communicating among the computers, and the strict agreement of communication prevents prevents the happening of mistakes and colliding of communication. Special measures are taken to keep the synchronism of the start, end and the process of welding. The controlling system has such advantages as high control function, easily operating, steadily working, high control precision, and completely meets the need of production technology.