Abstract: In order to consider the impacts of seismic demand dependence among structural components in the seismic fragility analysis of bridge system, a mixed Copula function is constructed by using Bayesian Model Average method, and then the dependence structure is separated from the marginal distribution functions of components. In combination with incremental dynamic analysis, the fragility curves and joint distribution functions of piers and bearings are developed. And a method for calculating the fragility of bridge system is proposed, which takes the seismic demand dependence among structural components into consideration. The results indicate that mixed Copula function can accurately depict the asymmetric dependence between the upper and lower tail distributions of the seismic demands of bridge components and simplify the modeling procedure of joint probability distribution function. A comparison between the system fragility curves obtained by the proposed method and those derived using Monte Carlo method shows good agreement. In addition, the computational efficiency is significantly improved by the mixed Copula function method because it significantly reduces the number of numerical samples.