The work aims to overlayAl-Si alloy coating on magnesium alloy, and analyze microstructure, composition and formation process of the transition zone on Al/Mg interface. A low-cost high-efficiency production process-direct current (DC) pulse metal inert gas (PMIG) shielded welding was used to deposit Al-Si alloy coating on AZ91D magnesium alloy using ER4043 wires (AlSi5). Microstructure, morphology and composition of the Mg/Al transition zone were measured with metallurgical microscope, scanning electron microscope, energy-dispersive spectrometer (EDS) and X-ray diffractometer(XRD). Formation process and composition of the transition zone were analyzedas well. The transition zone consisted of two regions. Region I was made up of Mg+Al12Mg17, Al12Mg17, Al12Mg17+Al3Mg2 and Al3Mg2, respectively. Irregular massive Mg2Si phase was dispersed in region I. The region II was mainly made up of columnar α-Al, with plenty of precipitated dotted Al3Mg2, and there was black dotted Mg2Si on grain boundary of columnar α-Al. Direct current pulse metal inert gas shielded welding can be used to prepare Al-Si coating on AZ91D magnesium alloy. There is a transition zone between substrate and coating. Relative Mg and Al content and composition are differentin positions of the transition zone.The precipitation of Mg2Si in region I and region II was different, which explained the different morphology. Mg2Si preferentially precipitated and grew tomassive Mg2Si in region I with relatively high magnesium content, while α-Al precipitated first in region II, and subsequently forms black dotted Mg2Si on grain boundary of α-Al.