Abstract: Daya Bay NPP has been operating safely and stably over 10 years since 1994, and its’seismic analysis of nuclear island was in accordance with the approaches in RCC-G standard for the model M310, in which the Simplified Impedance Matrix Method （SIMM） was employed for the consideration of SSI. Thanks to the rapid progress being made in upgrading the evaluation technology and the capability of data processing systems, methods and software tools for the SSI analysis have experienced significant development all over the world. Focused on the model of reactor building of the Daya Bay NPP, in this paper the more sophisticated 3D half-space continuum impedance method based on the Green functions is used to analyze the functions of the soil, and then the seismic responses of the coupled SSI system are calculated and compared with the corresponding design values. It demonstrates that the design method provides a set of conservatively safe results. The conclusions from the study is hopefully to provide some important references to the assessment of seismic safety margin for the operating NPPs.