The blade of large-scale wind turbine undergoes significant deflection and vibration during operation, which will impact the dynamic flow field around the blade and consequently alter the aerodynamic forces. Therefore, it is important to develop a deep understanding of the dynamic stall characteristics of airfoil undergoing complex motion, so that the operational loading of large-scale wind turbines can be accurately predicted. Applying computational fluid dynamics (CFD) techniques, this paper presents 2-dimensional numerical simulations of the dynamic stall characteristics of S809 airfoil undergoing different forms of motion. Firstly the dynamic stall behavior of the airfoil undergoing pitching motion in stall-development and deep-stall regimes is simulated using S-A, SST k-ω and RSM turbulence models. Comparisons with the experimental measurements indicate that all the three turbulence models can effectively predict the unsteady aerodynamic forces of the airfoil. Subsequently, the dynamic stall characteristics of the airfoil undergoing flapwise motion and combined pitching edgewise motion are simulated using SST k-ω model. The results are compared with those obtained by considering only pitching motion in the same condition. The dynamic stall of airfoil undergoing flapwise motion is weaker than that of airfoil undergoing pitching motion, but it is considerable and cannot be neglected. The dynamic stall of airfoil undergoing combined pitching edgewise motion is much stronger than that of airfoil undergoing pitching motion. The results suggest that in the design stage of a wind turbine, in order to obtain a conservative aerodynamic loading prediction, it is necessary to translate the motion of the blade cross-section in flapwise and edgewise directions into an equivalent angle of attack, and superimpose it on the main angle of attack to perform the dynamic stall calculation.