Abstract: Based on the study on Fire Probabilistic Safety Assessment（PSA） Methodology for Nuclear Power Facilities reported by NRC and EPRI,the methodology for estimating the fire frequencies resulting from various fire sources during the process of fire PSA is introduced,and an example is demonstrated by estimating the fire frequencies of the main transformer in Daya Bay nuclear power plant,which shows its fire frequency is 4.32×10^-3 per RY and this is 203 times of the core damage frequency of the reactor.It also shows that once a fire occurred on the transformer,the probability of catastrophic result would be 83%.