<p>To accurately assess the impact of initial defects in beam-column joints on the seismic performance of structures, this study introduces, for the first time, the initial defect depth (<i>a</i>₀) as a stochastic structural parameter into the seismic fragility assessment framework. Considering the variability of defects across different joints in practical engineering, a simplified approach assuming statistical independence of defect depths among joints is adopted. Firstly, a finite element model of a steel frame incorporating initial cracks at the joints is developed. Subsequently, a systematic parametric analysis is conducted to compare the seismic dynamic responses of the structure under varying initial crack depths. Building upon this, probabilistic seismic demand analysis is performed to establish the relationship between structural responses and ground motion intensity measures for steel frames with cracked joints. Fragility curves corresponding to different initial defect depths are generated, thereby quantifying the significant influence of these initial defects on both the seismic performance and collapse risk of the structure. The findings indicate that the modeling and analysis of steel frames with explicit consideration of initial joint defects enable a more realistic and effective assessment of structural seismic vulnerabilities. The derived fragility curves and parametric analysis results provide a theoretical basis for evaluating the seismic performance of existing structures.</p>