CFD is used to simulate the transient flow field of 5-blades vertical axis drag-type wind turbine (VAWT) with the rotor diameter of 4 m. Computational simulation on the change of moment coefficient of the wind turbine with time under different working conditions is used to analyze the dynamic characteristics of vertical axis wind turbines. The results show that the change of moment coefficient of flow field is cyclical, the average value and the cycle of moment coefficient decrease with the increase of rotational speed, while the amplitude increases. Its average value and amplitude rise sharply with the increase of wind speed. Meanwhile, its change is the periodic modulation wave. The wind speed has significant effect on the shape of moment coefficient curve. Moreover, the optimized rotational speed and the wind energy efficiency increase with the wind speed. As the wind speed increase from 7.5 to 9 m/s, the maximum wind energy efficiency increases from 23.2% to 25.8%, and the corresponding optimized rotational speed of wind turbine increases from 13 to 19 r/min.