Abstract: The causes of uneven size of spangle on the surface of hot dip Al-Zn steel sheet were analyzed by means of infrared spectrum analysis, scanning electron microscope and energy spectrum analysis. The results show that the uneven size of spangle was caused by the difference of iron dissolution in the steel substrate. During immersion plating, a sufficient amount of iron element was dissolved in the surface layer of the normal steel substrate. The supersaturated iron element limited the nucleation rate of aluminum dendrites, and promoted the distribution of silicon element, there were fewer silicon needles precipitated from eutectic. When the outer coating was solidified, the aluminum dendrites had fewer nuclei, and grew into normal large spangles. The substrate with iron carboxylate on the surface had poor wettability, less iron dissolution, unrestricted nucleation of aluminum dendrites and small spangles were formed.