Abstract: Hot-dipped galvalum steel plates are widely used because of the excellent corrosion resistance.The production technology of galvalum steel plates is very mature,but the control of the surface quality of products (especially the thick specifications) is a difficult production technology.Bump defect is one of the main drawbacks for galvalum steel plates.The microstructure of the surface and section of the bump defects was analyzed by scanning electron microscopy(SEM) equipped with EDS.The results showed that the bump defect was formed during hot-dip,and one of important reasons for the formation of bump defect was the slag accumulation of sinking roller.The defect can be reduced effectively by process optimization.