Abstract: The vertical and rotational motion equations of a bridge deck section model elastically suspended in wind tunnel about its mass center are formulated using Lagrangian equation. This mathematics model considers the elasticity eccentricity and the damping eccentricity. Subsection Extended Order Iterative Least Square (SEO-ILS) algorithm is developed for direct identification of system matrix from free vibration data of the bridge deck sectional model in wind tunnel test. The method for identifying mechanical modal parameter and physical parameter of the section model in zero wind case is presented considering the eccentricities. A thin plate and a bridge deck section model were tested in wind tunnel and the eight aerodynamic derivatives were estimated by the presented method. The results indicate the reliability and effectiveness of the proposed method.