Abstract: Almost the complete wall thickness reduction is finished at the zone of the hump on the roll of the 3-roll mill, which means that the hump has so strong elongation ability that the deformation as conducted on the hump is not only large, but also so concentrated and fast. Based on the research on the said deformation characteristics, relevant technological issues are discussed such as deformation amount, rolling of hi-alloy steel and the rolling tolerance of light wall tube. What is resulted from the said research is worth using as reference for tube makers'selection of the mill type.