Reliability allocation plays an important role in the forward risk-informed design. At present, there is no clear method in the nuclear industry to obtain the reliability target of each frontier system from the core damage frequency (CDF) index. Based on the previous experience of probabilistic safety analysis (PSA), this paper describes a reliability allocation method applicable to nuclear power plant level to system function level. The reliability allocation work from plant level to system function level is carried out based on CDF index of internal event level one. By establishing CDF allocation model, functional reliability index of each frontier system is obtained. It lays a foundation for the subsequent reliability distribution to the equipment level, and provides reliability support for the system design of new nuclear power units.