In order to repair thin-walled castings of ZM5 magnesium alloy, the parameters for fiber laser welding are determined by dilution rate and weld-surface angle. Scanning electron microscope (SEM) and X-ray diffraction (XRD) are adopted for microstructure observation and analysis of chemical composition while tensile tests are carried out for evaluation of mechanical properties. It′s demonstrated that the relationship between dilution rate and weld-surface angle is linear dependence. The fiber laser welding layer consists of tiny equiaxed dendrites with the average grain diameter of 15 to 20 μm which is 1/4 of ZM5 base. The base structure of welding layer is α - Mg phase, and the eutectic Mg-Al phase, β - Mg17Al12 mainly, is even dispersed along the grain boundary of welding layer. Shown in tensile tests, quasi-cleavage fracture occurred in fiber laser welding layer, which achieves higher strength and toughness than those of the brittle-fractured base metal. Thus thin-walled castings of ZM5 magnesium alloy are so well repaired by fiber laser that production quality and efficiency are greatly improved.