On the basis of the processes of fire happened in some sewage treatment plants, by reasonably choosing parameters, e.g., fire time, fire temperature, heat capacity of concrete, concrete thermal conductivity, air temperature, intensity of heat exchange of concrete surface, a model is established to obtain the temperature field of structure. According to chemical stability and thermodynamic properties of concrete, steel and pre-stressed strands under different temperature actions, an analysis of structure safety is carried out and numerical results coincide with conclusions drawn from tests after fire. The method provided is suitable for evaluation on structure safety after fire.