Based on the recent practical work of evaluation of seismic safety for the nuclear power plant site selection and construction in China, some key issues were discussed involved in the determination of site specific design ground motion for nuclear power plants, including the seismic hazard analysis methods, the ground motion attenuation relations, and the methods of determining design ground motion parameter. The countermeasure change was analyzed to consider the uncertainty of ground motion attenuation relation and the ground motion attenuation relation of diffuse earthquake; the differences were discussed between the computed results from the probabilistic method and the approaches of the diffuse earthquakes and tectonic earthquakes in the deterministic method, and their controlling effects were also discussed on the determination of site specific design ground motion. The results indicated that: a. comprehensive consideration of seismic hazard analysis results from the deterministic method and probabilistic method is the basic ideas of determining design ground motion parameters in the evaluation of seismic safety for nuclear power plant sites, in which the diffuse earthquake is a special problem considered; b. the controlling effects of the computed results from the probabilistic method and deterministic method on the determination of site specific design ground motion are affected by the seismicity in the study area; c. in the weaker seismicity areas, site specific design ground motion parameters are mostly controlled by the computed results from the deterministic method, especially from the approach of diffuse earthquake, but in relatively strong seismicity areas, site specific design ground motion parameters are more likely controlled by the computed results from the probabilistic method.