Abstract: As a high-order statically indeterminate structure, long-span cable-stayed bridges usually consist of many components such as main towers, stay cables, main beams, auxiliary piers, and connecting piers. Due to the mutual influence between components under earthquakes, the accurate simulations of the correlations among component seismic responses are critical to the vulnerability assessment of the overall system of a cable-stayed bridge. Pair Copula can simulate the correlation between two components. It is theoretically feasible to use Pair Copula in a hierarchically iterated way to simulate the whole system of a cable-stayed bridge. Therefore, a new seismic vulnerability assessment method of a cable-stayed bridge system is proposed based on Pair Copula iterative model. Based on structural uncertainty parameters and ground motion uncertainty, Latin hypercube sampling technique is used to establish bridge-ground motion probabilistic seismic response analysis samples. The correlations among the seismic responses of components are quantified through nonlinear dynamic time history and correlation analysis. Pair Copula models are fitted with maximum likelihood estimation and optimized based on AIC and BIC criteria. Through the hierarchical iteration of Pair Copulas, the overall model of a cable-stayed bridge is established, and its seismic vulnerability is evaluated. The engineering example shows that the correlation among multiple components can be accurately simulated based on the technology of hierarchical iteration of Pair Copula. With the assumption that the seismic responses of components are completely unrelated, the seismic vulnerability of a whole cable-stayed bridge system will be significantly overestimated.