Accumulator middle tread run is welded by submerged arc welding for HL1000 nuclear plant in China. The welding is performed with the combination of sphere positioner and narrow gap submerged arc welding machine,and the automation welding of sphere arc-shaped curve welds of ACC middle tread run is realized. The influence factors for welding quality are discussed from material,welding current,voltage,rolling speed of the positioner and weld groove type. The measures are adopted from the design of groove,the selection of welding methods,the control of the size of middle tread run,the control of assembly size,the control of welding process,the welding sequence and so on,and the technology planning is done in all directions. Finally,the welding of three accumulator middle tread runs for HL1000 nuclear plant is accomplished.