To reveal the reason of weld-line movement in hydroforming of a tailor-welded tube (TWT) with dissimilar thickness,the stress ratio of axial stress to circumferential stress is derived by mechanical analysis and analyzed between the thicker and thinner tubes,as well as the property of the axial strain.During TWT hydroforming,tensile strain along axial direction happens on the thinner tube.On the contrary,compressive strain happens on the thicker tube.Experiments are conducted to varify the weld-line movement regularity and strain distribution.It indicates that the weld-line moves from the thinner part to the thicker during TWT hydroforming.The thinning ratio of the thinner tube is bigger than that of the thicker tube,especially in the zone near weldline.Stress ratio difference between the thicker tube and the thinner tube is the main reason of weld-line movement and non-uniform thinning ratio distribution.