Abstract: The expander tool is a key part of the mechanical expander for the production of the longitudinal submerged-arc welded pipes.Usually it needs to order from a foreign company with very high prices and long delivery time.So it is urgent to develop the home-made one.The deferent structures and lubrications of the middle and large diameter tools are analyzed starting from its technological difficulties.The deferent tool steels and the related heat treatment processes are used to ensure its integrated service life regarding to its using conditions in the design of its structure.More than 30,000 lengths of the longitudinal submerged-arc welded pipes have been produced all of them have meet the expanding requirement with 100% qualified since the newly developed tool was put into use two years ago.