The micro-defects on the surface of VW63Z rare earth magnesium alloy castings were shown as "stripe fluorescence" as a result of fluorescence detection. The micro-defects which cannot be removed by mechanical processing would directly scrap the castings if it appeared on the non-machined surface of the casting. The microstructure of the surface defects of castings and their formation mechanism were investigated in this paper. Results showed that the main component of the micro defects was the double-layer oxide film of rare earth oxide， some of which contained inclusions； the confluence of the molten metal made the surface micro defects to form more easily.The laser melting technology was used to treat the surface micro-defects of the casting. When the output power of the flat-top laser beam was 800 W， the microstructure of the melted layer presented the dendrite characteristic， had finer grains and good combination with the matrix. The depth of the remelted layer was about 915 μm， which could basically repair the surface defects of the casting while controlling the heat-affected range of the casting. The fluorescence detection of the original defect position showed no streak defects.