This study focuses on investigating the impact of varying fire durations on the seismic performance of steel frame structures, aiming to provide a scientific basis for improving their post-fire seismic performance. PyroSim fire simulation software was utilized to calculate the temperature rise curves in the fire-exposed regions of steel frame structures. These calculations were integrated with post-fire material strength reduction formulas from existing literature to establish a finite element model of fire-damaged steel frame structures using ABAQUS. Incremental dynamic analysis (IDA) was employed to conduct seismic vulnerability assessments of the model, and a probabilistic seismic demand model was developed to evaluate its seismic performance across different damage states. Based on the findings, for fire-damaged steel frame structures failing to meet national standards, two retrofitting schemes, X-braced and M-braced systems, were proposed. The seismic vulnerabilities of the structures before and after retrofitting were compared. The results indicate that fire significantly increases the seismic vulnerability of steel frame structures, with a notable rise in exceedance probabilities across various damage states. After retrofitting, the structural performance met national standards, satisfying safety and serviceability requirements. By systematically analyzing the changes in seismic vulnerability of steel frame structures before and after fire exposure, this study reveals the deterioration mechanisms of seismic performance due to fire. It provides valuable insights and practical references for the seismic safety evaluation and retrofitting design of fire-exposed steel structures.