Abstract: In order to study the effects of spatial coherency on the seismic response of nuclear island structures considering soil structure interaction analyses, in this paper, ACS-SASSI software was used to analyze the soil structure interaction of a typical PWR reactor building. Considering different soil layers and elevations, the effects of spatial coherency on the seismic floor response spectrum of nuclear island structures were evaluated. The results show that at high frequency the floor response spectrum will be reduced by 10%~70% for firm rock and Upper Bound Soft to Medium site. For soft soil sites, it will be reduced by 10%~40%. Thus, the floor response spectrum is conservative at high frequency and is not safe at low frequency without considering the spatial coherency.