Abstract: Through experiments of hot-dipping 55Al-Zn-1.5Si alloy coating on the steel sheet,the effect of plating auxiliary and hot-dip processing parameters on coating thickness and quality were studied.The results show that the plating auxiliary had less effect on the alloy layer thickness,but influenced the anticorrosion of Al-Zn alloy coating directly.The optimized composition of water-soluble auxiliary for 55Al-Zn-1.5Si alloy coating was 27%ZnCl2+ 10%NH4Cl.When the hot dip temperature was 640-660 ℃,the hdding time was 6 minute and the raising speed was 5 m·min-1,coatings with high quality could be gained.