Objective: Steel frame structures are highly vulnerable to collapse during fire, posing severe threats to occupant safety and property protection. As reliable methods for assessing structural performance under fire exposure, fire tests are essential for elucidating structural response mechanisms. However, constrained by budgetary limitations and logistical challenges, most contemporary fire tests on steel frame structures primarily focus on evaluating structural integrity under fire conditions, rarely progressing to the collapse stage. This limitation hinders a comprehensive understanding of steel structure collapse mechanisms under fire, emphasizing the urgent need for systematic investigation. Methods: To explore the collapse evolution mechanisms of steel frame structures under real-fire scenarios, a reduced scale experimental fire test was conducted on a two-story, three-span steel frame structure, subjecting the bottom-floor compartment to fire exposure. Real-time data were collected, including temperature distribution within the structure, thermal responses of critical structural components, and three-dimensional displacement behavior of the structure. The experimental process was meticulously documented, capturing the entire progression from fire ignition to structural collapse. Results: The key findings of the test were the following: (1) The localized strong ventilation effect induced by the open doorway significantly accelerated the combustion rate of the wooden crib in the central room among the three rooms side by side, leading to asymmetric fire propagation from the central room toward adjacent areas; (2) The test frame experienced global collapse at 82 minutes after fire exposure, with the maximum structural temperature reaching approximately 900 ℃ at the time of collapse. The collapse occurred in three stages: Initial stage (0-45 min), where internal steel columns buckled under combined axial loading and rapid heating (12 ℃/min); Progressive stage (45-68 min), where load redistribution triggered a sharp increase in the axial forces of adjacent columns; and Final stage (68-82 min), where a progressive downward collapse occurred in the interior span, characterized by localized sunken failure. Conclusions: By analyzing the test results, the following conclusions are drawn: (1) The test validates that the failure of steel columns is the principal factor governing the fire-induced collapse of redundant steel frame structures. Furthermore, it elucidates the collapse mechanisms, which are driven by the sequential failure of critical structural components. (2) The observed fire development highlights a significant correlation between the fire propagation path within the building and ventilation conditions. When designing building ventilation systems, the influence of ventilation on fire field evolution must be thoroughly considered to reduce the potential for accelerated structural failure caused by localized strong ventilation effects. (3) The test reveals that the fire-resistance performance of the overall structural system surpasses that of individual components. To enhance fire-resistance design, structural redundancy should be strategically leveraged to mitigate vulnerabilities associated with a single load-transfer pathway. (4) The collapse mode of the tested steel frame exhibits typical characteristics of progressive sunken failure. Each buckling of internal steel columns is identified as a critical indicator for collapse warning.