Globally, multi-units are located at the same nuclear power plant site. After several serious nuclear accidents, the overall risk assessment of a nuclear power plant with multiple reactors has attracted much attention. Combined with the concept of PSA (probabilistic safety analysis) for multi-units, this paper analyzes the off-plant power supply configuration of 9 units in Qinshan Nuclear Power Base. In view of the 6KV emergency busbar directly affected by the instability of the off-site power grid, the correlation among several units is fully considered, and the exploratory research and analysis of off-site power grid reliability is carried out in Qinshan Nuclear Power Base. Taking the reliability of external power grid of Qinshan Nuclear Power Base as an example, the reliability of emergency bus and the influence range and contribution of various analysis factors to the evaluation results are quantitatively analyzed through fault tree modeling. The results of model analysis are in line with the actual situation. The analysis concept based on the multi-unit PSA methodology should be used in a wider range. For example, thematic analysis should be carried out for other highly correlated originating events (common cold source, equipment common cause failure, etc.), and the development and progress of multi-unit PSA should be promoted through practice with a point-to-surface approach. In the follow-up research, we should improve the model system, take the actual situation into full consideration, and develop a set of overall risk assessment system that can reflect the real-time risk level of multiple unit site.