Abstract: The water treatment plant system is a crucial component of urban water supply systems. Seismic damage to the internal water treatment structures, buildings and other components within the plant area can significantly impact the post-earthquake water supply capacity of the system. Existing research lacks a comprehensive model and methodology for quantifying the post-earthquake functional of water treatment plant systems. This study proposes a model for assessing the post-earthquake water supply capacity of the plant, considering the post-seismic state of different components within the plant. Considering the seismic vulnerability of water treatment structures, buildings and connecting pipelines involved in the water treatment process, a state tree model is employed to represent the interconnectedness among various components and their influence on the water treatment plant. The model integrates the seismic vulnerability assessment of various components through Monte Carlo simulation, calculating the probabilities of different damage states. By utilizing logical gates within the state tree, the influence of component damage states on the water supply capacity of the plant system is determined. The proposed model is validated by assessing the post-earthquake service capacity of a realistic water treatment plant system. The results demonstrate that the seismic vulnerability of the plant system differs significantly from individual structures. Therefore, relying solely on the seismic vulnerability of individual structures cannot adequately evaluate the post-earthquake functional state of the plant system. The assessment model's findings align well with the actual seismic damage investigation reports on post-earthquake functional state of the plant system.