Numerical simulation on unsteady rotated flow of a straight-blade vertical axis wind turbine (VAWT) is done based on moving meshes by means of the software Fluent. The wake velocity and the variation of velocity at the special cross-section are obtained. The results of unsteady flow are compared with ones solved by the multiple stream-tube model, and good coincidence is obtained. The conclusion can be made that the numerical simulation on unsteady rotated flow of a straight-blade VAWT based on moving meshes can represent its flow field. 