Objective This paper aims to explore the causes and formation mechanism of spot defects on the surface of ZM6 magnesium alloy. Methods Spot defects with different morphologies on the surfaces of unmachined and machined ZM6 magnesium alloys were analyzed by scanning electron microscopy SEM . Simultaneously the chemical compositions of these spot defects were identified by energy dispersive X-ray spectroscopy EDS . Finally a comprehensive analysis was carried out combining the as-cast microstructure and mechanical properties of the alloy. Results The results showed that spot defects on the surface of the unmachined ZM6 magnesium alloy were deep black while the spot defects on the surface of the machined ZM6 magnesium alloy were white and there was a small black area in the center of some white spots. The spot defects on the surfaces of both unmachined and machined ZM6 magnesium alloys were cracked and no voids were found at the transition interface with the substrate. The main components were oxides or hydroxides of Mg while also containing impurity elements such as Cl and K introduced by the solvent. The content of the Cl element was relatively high in the surface spot defects of unmachined ZM6 Mg alloy while the content of the K element was relatively low in the surface spot defects of machined ZM6 Mg alloy. It?? s worth noting that there were tiny pores in the black area of the white spots on the surface of the machined ZM6 magnesium alloy. Conclusions It can be concluded that the spot defects on the surface of ZM6 magnesium alloy are caused by chemical and galvanic corrosion of Mg rather than by oxidation inclusions. Therefore strengthening the refining effect of ZM6 casting magnesium alloy to obtain a melt with uniform chemical composition is an effective way to obtain high-quality components.