Abstract: Elaborated here in the article are the characteristics of the modification project for a certain domestic Φ89 mm mandrel mill line.In order to fulfill the construction task of the said project with high efficiency and high quality,during the modification design of the engineering drawings of the said mandrel mill line,couple of methods are adopted,including the 3-D-aided design,the optimal process design,and deep integration of design & construction,etc.Compared with similar mill lines in the country,owing to the modification project,this mandrel mill line is highlighted in many aspects,and produces both remarkable economic profit and social profit,having been a very good example for upgrading small-sized seamless steel pipe mills.