Currently, multi-unit probabilistic safety assessment (PSA) models have been actively developed. By adopting integration method, a large model has been built up to perform multi-unit level 1 PSA while inter-unit component correlation or seismic correlation is still applied through various assumptions, or methodology is being developed to derive its parameters. Therefore, a simplified method for deriving multi-unit conditional core damage probability (CCDP) using Bayesian belief network (BBN) is proposed. According to the methodology, the target system or unit is analyzed. Selected components through importance analysis are modeled using BBN. Moreover, seismic failure probability derived from seismic fragility and inter-unit correlation are applied as input data. As a result, the BBN model deducts multi-unit CCDP according to various inter-unit correlation. In this study, for the uncertainty analysis of simplified method, the failure probability and the inter-unit correlation are expressed by distributions to derive the multi-unit CCDP. The results of the distribution reflect uncertainty and the proportion of CCDPs that exceed the reference CCDP is derived. Therefore, the proposed simplified multi-unit CCDP methodology is able to complement the multi-unit PSA results due to the lack of data and various assumptions by reflecting uncertainty. Thus, this is expected to be used as a kind of pre-assessment before developing a complex full model, or it can be used for public communication by using an intuitive and simple advantage.