Abstract: The overlaying welding was produced on the surface of AZ91D magnesium alloy by electro-spark welding(ESW) technique using an electrode which is as same as the base metal. The microstructure, bonding interface and corrosion resistance of the layer were investigated. The results showed that the ESW process can produce a uniform and dense weld with proper process parameters. The weld microstructure consisted of supersaturated α-Mg solid solution with size of 1-5 μm, Mg₁₇Al₁₂ phase and metastable AlMg phase. The weld was metallurgically bonded to the base metal and a thin layer formed by interdiffusion, crystallization was formed at the interface between the weld and base metal. There is no obvious heat affected zone on the side of the base metal and original composition of the electrode material was retained in the weld. The corrosion resistance of weld was better than the base metal. The grain refinement of the weld can improve the uniformity of the corrosion, meanwhile, the supersaturated α-Mg solid solution and β phase that was continuously distributed at the grain boundaries with the a netlike morphology can decrease the corrosion rate, which are the main factors leading to a high corrosion resistance of the weld.