Abstract: As for most of the nuclear power plants worldwide, both the probabilistic risk assessments (PRA) and the safety goals are from a single reactor unit basis. In fact, the sites with multiple plants, as well as the plants housing multiple reactors, are definitely not rare. This study is aimed at bringing the PRA scope from the “single reactor” to the “whole site”. With the site risk definition clarified, the initiating events (IEs) and proposed recommendations on classification and identification were analyzed in the paper. According to the results, the initiating events for the multi-unit sites can be categorized as two single-reactor IEs (SRIE) and three multi-reactor IEs (MRIE). This study is of great value as the first fundamental step to conduct the PRA for the multi-unit nuclear power sites.