Abstract: The MgNdGdZnZr bisemiconductor rare-earth heat-resistant magnesium alloy exhibits both high specific strength and excellent casting properties, making it a key material for lightweight aerospace components. However, casting defects can readily lead to fatigue failure, limiting its engineering applications. To compare the fatigue performance of laser beam welding (LBW) and friction stir welding (FSW) in repairing defects in MgNdGdZnZr dual-rare-earth magnesium alloy, cast-state MgNdGdZnZr was used as the test material. Four-point bending fatigue tests were conducted, and fatigue life and S-N curves were obtained using both the staircase method and the group test method. Microstructural features and fatigue fracture morphologies were characterized by means of OM, EBSD, SEM, and TEM. The results reveal that the FSW weld features fine, uniform equiaxed crystals, resulting in significant plastic deformation of the material, the grains in the heat-affected zone are elongated, while the second phase is dispersed without notable segregation, whereas the LBW heat-affected zone exhibits pronounced grain coarsening and segregation of rare-earth phases. The fatigue limit of the FSW joint is 148 MPa, slightly lower than that of the LBW joint (157.5 MPa), yet the coefficient of variation for FSW (4.8%) is higher than that for LBW (2.3%). At high stress amplitudes, fatigue behavior is governed by crack initiation. FSW is prone to introduce sharp geometric discontinuities that accelerate crack nucleation. At low stress amplitudes, fatigue is dominated by crack propagation, during which the fine-grained structure characteristic of FSW effectively impedes crack growth. The LBW fracture surface is generally rough and uneven, with scattered secondary phase particles, oxide inclusions, and micropores on its surface. The FSW fracture exhibits typical fatigue fracture characteristics, its cross-section consists of multiple parallel fatigue extension platforms separated by distinct tearing ridges and crack steps, presenting a relatively smooth overall morphology with numerous directionally arranged fatigue striations distributed across the surface.Based on the classical Hall-Petch relationship, the effects of precipitate strengthening and grain boundary segregation weakening are incorporated.Through multivariate linear combination and dimensionless processing, and by accounting for variations in microstructure, welding defect characteristics, and crack initiation/propagation mechanisms, a quantitative fatigue limit model is constructed and refined. The resulting multi-mechanism coupled fatigue limit model achieves a goodness-of-fit R²>0.98 with a prediction error of less than 0.5%. This investigation provides data support for optimizing welding repair strategies for rare-earth magnesium alloy components in aerospace applications.