In this paper, we propose a probabilistic method for analysing the collapse time of steel frame structures in a fire. The method considers the uncertainty of influencing factors. Tornado diagrams are used for sensitivity analysis of random variables. Structural analysis samples are selected by Monte Carlo method, and the collapse times of different structural samples are calculated by fire time history analysis. A collapse time fragility curve is fitted according to the calculated collapse times of the samples. A reliability index of the collapse time is used as a quantitative standard to evaluate the collapse performance of a steel frame in a fire. Finally, this method is applied to analyse the collapse time fragility of an eight-storey 3D steel frame structure under different compartment fire scenarios and fire protection levels. According to the collapse time fragility curve, the effects of the different fire scenarios and protection levels on the collapse resistance of the structure under fire are evaluated.