In this paper, the natural characteristic of the flexible blade of Horizontal Axis Wind Turbine in flapwise/edgewise and torsion directions was obtained, based on modal analysis theory, through two-node cantilever beam discrete element analysis. The analysis of structure dynamic response was achieved using constant average acceleration method of Newmark and uncoupling every mode of the vibration. Take a 5MW HAWT for example, the effect of higher-order characteristic modes on the dynamic response of the blade was analyzed. Furthermore, the variation of aerofoil section twist angle and the feedback effects on the aeroelasticity loads was considered. The results showed that the flexible blade of large wind turbines subjects to significant vibration and deformation in the process of rotating; and the aerofoil section torsion change has a great influence on the aerodynamic characteristic of the large scale wind turbines.