1. A single quantification method of an external event probabilistic safety assessment (PSA) model containing multi-compartment scenarios, comprising: loading an internal event PSA logic model having core damage as a top event; constituting a mapping table comprising external events containing the multi-compartment scenarios in consideration of information regarding external event occurrence frequencies, external event-induced initiators, and equipments damaged by external events; reflecting the mapping table in the internal event PSA logic model to establish an external event PSA model; calculating a final minimum cut set (MCS) based on the external event PSA model; and calculating a core damage frequency (CDF) value according to the final MCS.