Abstract: After the Fukushima nuclear accident, more and more attentions were paid to the integrated risk of multi-unit nuclear power plant (NPP), while there is no any methodology or guideline, at home or abroad on assessing the integrated risk of multi-unit NPP sites. According to the general requirements for NPP sites by relevant regulations, an exploration was developed to assess the integrated risk of the multi-unit site by extending the methodology of level-1 probabilistic safety assessment (PSA) of the single-unit site. A two-unit site was employed as a demonstration to discuss some quantification issues of multi-unit sites, with some insights on dependencies among the units and mathematical mechanisms. The approach discussed in this paper is of important value for the research on PSA methods of multi-unit NPPs.