Abstract: Due to the uncertainty of earthquake, the possibility of earthquake exceeding the design level may occure, so it is necessary to master the capacity of nuclear power plants to resist earthquake exceeding the design level. The aseismic margin analysis method was used internationally to evaluate the bearing margin of nuclear power plants subjected to earthquakes beyond the design level. The SFA method was an effective calculation method with the high degree of confidence and low probability of the failure value (HCLPF value) for nuclear power plants, in which the median aseismic capacity and uncertainty coefficient of the structure, system and members were calculated by the SFA according to obtained data, and then the HCLPF value was also calculated. A calculation method of HCLPF based on non-central distribution t was proposed, which did not need to assume the distribution of cognitive uncertainty, to avoid the problem that the value of relevant parameters of cognitive uncertainty had a great impact on the calculation results of HCLPF.