Based on the Beddoes-Leishman (B-L) semi-empirical dynamic stall model, the dynamic stall characteristics of wind turbine airfoils were studied through the analysis of attached flows, separated flows and dynamic vortices. According to the operating conditions of wind turbine airfoils, the method for evaluating dynamic stall coefficients associated with the separated flows and dynamic vortices in the B-L model was amended. The amended model was also extended to produce transient aerodynamic force coefficients over the entire range of possible angles of attack. The lift, drag and pitch moment coefficients of wind turbine airfoils S809 and NACA 4415 during dynamic stall were calculated. The simulation results showed good agreement with experimental data. Compared with the original B-L model, the present model shows considerable improvements in accuracy when predicting the transient aerodynamic force coefficients and is capable of producing better dynamic stall estimations.