Abstract: Repair welding is an important means to improve the performance and quality of WE43A magnesium alloy castings. In this study, the stereo microscope, metallographic microscope, scanning electron microscope and its attached energy dispersive spectrometer, were used to analyze the structure of the repaired joint. The microhardness tester was used to test the hardness change of the joint. Also, the fatigue fracture of the repaired joint was analyzed at the same time. The results show that there is no obvious heat-affected zone after repair welding of WE43A magnesium alloy, and square Mg₂₄Y₅ phase and precipitation strengthening phase Mg₁₄NdY, Mg₃(Y,Nd) can be seen in the base metal. In the weld zone, the grains are equiaxed, and their size increases gradually from the fusion line to the central grain. The precipitation phases are mainly Mg₂₄Y₅, Mg₁₄Nd₂Y and Mg₃(Y,Nd). The microhardness of the base metal and the weld on both sides of the fusion line is uniform, and the weld zone is 8.13% lower than the base metal. This is due to the precipitation of a large amount of second phase in the weld area, which reduces the solid solubility of the matrix, and the precipitation of Mg₃(Y,Nd) phase is coarse and does not produce precipitation strengthening comprehensively. The source region and extension region of the fatigue fracture of the repaired joint have the features of quasi-cleavage. There are many fan-shaped patterns in the source region, and a large number of steps and a small number of intergranular features in the extension region. There is a mixed fracture feature of intergranular and dimples in the transient fracture region.