Fire accidents caused by the leakage of combustible gas pose a serious threat to the offshore platform, probably resulting in local damage and even progressive collapse of the platform structure. The aim of this paper is to systematically assess the structure safety of platform under fire accidents. The uniqueness of this study is the integration of fluid-thermal-structural coupling simulations with the advantage of considering an accidental fire scenario. The dispersion behavior of leaked gas is studied and the development of elevated temperature generated on the offshore platform during the combustion process is predicted based on Computational Fluid Dynamics (CFD)- Finite Element Analysis (FEA) coupling method. Then, the thermal-mechanical coupling analysis is performed to predict the responses of structure under high temperature. Eventually, the criteria of ultimate bearing capacity of the component and the overall offshore structure are utilized to evaluate the safety of the offshore platform. The results show that with the development of fire, the high temperature zone spreads to cover the space between two neighboring decks, and the maximum temperature in the platform reaches 877 °C. The structural strength of the platform is significantly affected by high temperature. Under the vertical load, the ultimate bearing capacity of the platform at high temperature is decreased by 78% compared to that at ambient temperature. The methodology proposed could be applied to the safety assessment of other similar offshore or marine facilities, so as to support to the process safety in fire accidents.