Abstract: In this paper the reason for falling-off of welding iron oxide scale in pickle sheet had been investigated by comparing and analyzing chemical composition,microstructure,iron oxide structure,bath depth and sheet thickness of Hansteel’s sheet for Compressor.It is found out that the thickness of iron oxide scale of Hansteel is thicker than Baosteel and the proportion of the outer layer of Fe₃O₄ is higher than Baosteel. The difference in the thickness of Fe₃O₄ directly causes the falling-off of welding iron oxide scale of Hansteel.The content of C in SPHD-YSJ is low,leading to the iron oxide scale growth more rapidly during the welding process and thicker iron oxide scale,which is beneficial to the formation of Fe₃O₄.All those are the essential for the falling-off of the iron oxide scale.When increasing C content to 0.05%-0.07% and controlling the thickness based on micro-negative tolerance or zero tolerance,the welded iron oxide scale adhesion are improved and the problem of falling-off of the iron oxide scale has been successfully eliminated.