A systematic investigation was conducted on an advanced weld repair methodology aimed at rectifying castingdefects in critical components, specifically brake calliper brackets manufactured from G20Mn5QT low-alloy cast steel,which are employed in rail transit systems. The primary goal is to improve the quality of defect remediation and enhancethe operational reliability of these components. G20Mn5QT, a low-alloy cast steel widely used in vital load-bearingstructures such as bogies and braking mechanisms, is prone to casting imperfections, including porosity, slag inclusions, andcracking, largely attributable to process variability. These defects substantially undermine the service life and safetyperformance of the components. Welding repair constitutes a pivotal approach to address these flaws, thereby loweringproduction expenses and optimizing material usage. However, the relatively elevated carbon equivalent of G20Mn5QT(ranging from 0.48~0.64), coupled with its predominantly martensitic microstructure, predisposes the welding process tochallenges such as embrittlement within the heat-affected zone, cold cracking, and residual stress accumulation,necessitating stringent control over the welding parameters. On this basis, shielded metal arc welding (SMAW) utilizing thelow-hydrogen, high-toughness electrode ESAB OK48.04 was employed to systematically evaluate the effects of criticalwelding parameters, including preheating temperature, interpass temperature, and heat input, on the microstructural characteristics and mechanical performance of the welded joints. Comprehensive assessments were conducted throughmechanical testing, hardness profiling, and metallographic examination. The findings reveal that under optimized conditions,specifically, preheating at (200±10) ℃, interpassing temperatures between 210 and 300 ℃, and heat inputs ranging from1.0 to 1.8 kJ/mm, the weld repairs achieve tensile strengths exceeding 571 MPa, fracture elongations of no less than27% , and Charpy impact energies at -40 ℃ reaching up to 130 J. The hardness distribution across the weld metal andheat-affected zone is uniform, with continuous microstructural transitions and an absence of defects such as cracks orporosity. The optimization of the weld repair process facilitates the high-quality restoration of casting defects in G20Mn5QTcast steel, yielding joint strengths comparable to those of the base material and significantly improving the low-temperaturetoughness.