Abstract: Calculation of the high confidence of low probability of failure（HCLPF） value of the structure, system and component（SSC） in nuclear power plants is an important process for seismic margin assessment（SMA）. In this paper, four methods for calculation of the HCLPF value of SSC are presented, such as, fragility analysis, conservative deterministic failure margin（CDFM）, HCLPF value based on testing, and the nonlinear time-history method. Subsequently based on the pressurizer model, the HCLPF of the supports of the pressurizer is obtained by nonlinear time history analysis and the CDFM method. The results show that two methods are different. The value of CDFM method is more conservative and the value of nonlinear time history method is more reasonable. It is suggested that the nonlinear time-history method is adopted to obtain the seismic capacity of nonlinear equipments.