Abstract: A full automatic welding machine for spherical tanks all position mutil-layer welds has been developed. This machine is mainly composed of a two-dimension seam tracking system based on microcom puter's memory and a welding tractor as well as rail. The main features of the machine are:while welding the first layer of a seam, its microcomputer system can analyze and store the tracing information from a two-dimension sensor, and control the welding head device to realize two-dimension real time seam tracking; while welding the second layer up to the top layer of the seam, it can realize two-dimension real time seam tracking memo rial tracking based on the memo rial data, automatically determine the layer number and continually sway the welding head. The welding test shows that the machine has good tracing and welding behavior, and is suitable for spherical tank's all position mutillayer welds.