Abstract: Objective This study aims to investigate microstructure and mechanical properties of Mg-Gd-Nd-Zr magnesium alloy plates repaired by passive filling friction stir repairing (PFFSR), in order to provide theoretical reference for engineering applications. Methods In this paper, PFFSR was employed to repair large-size defects (with diameters of 10 mm, 20 mm, and 40 mm, and a depth of 5 mm) in Mg-Gd-Nd-Zr magnesium alloy. Microstructure of the repaired samples was characterized by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and electron backscatter diffraction. Mechanical properties were evaluated through microhardness testing and universal testing machine measurements. Results The results demonstrate that PFFSR effectively eliminates defects across three different sizes, achieving a dense metallurgical bonding between repaired zone and base plate. The coupling effect of severe plastic deformation and dynamic recrystallization significantly refines grain structure and promotes dispersion of the second phase. The average grain size is refined from 39.2 μm in base metal to 2.1 μm, leading to notable grain refinement strengthening in the repaired zone. However, grain growth and coarsening of the second phase are observed at overlapping interfaces due to repeated PFFSR thermal–mechanical cycles. The average microhardness of the repaired zone reaches 73 HV. After repairing a 10 mm diameter defect, tensile strength reaches 224 MPa, representing 85.2% that of base metal, while elongation increases significantly to 13.2%, 240% that of base metal. With increasing defect size (20 mm and 40 mm), prolonged processing and higher heat input results in a decrease in elongation to 9.4% and 7.7%, respectively, though tensile strength remains stable. Conclusion PFFSR can achieve tensile strength over 85% that of base metal after repairing three types of large-sized defects, providing an efficient and reliable method for repairing magnesium alloy components.