A constructed one-zone inhomogeneous phenomenological model of Nanosecond Dielectric Barrier Discharge (NSDBD) is coupled with the unsteady Navier-Stokes equations to model the effects of nanosecond plasma actuation on flowfield. The coupled system is solved by using unsteady Reynolds-Averaged Navier-Stokes equations (URANS) which can predict the compression-expansion wave structures and wave speed well compared with experimental results and can be applied to simulate the flow control by using NSDBD. The model is adopted to investigate the separation control airfoil using NSDBD plasma actuator. The separation-control mechanisms over NACA0015 are investigated by using URANS and Large-Eddy Simulation (LES). It is found that the transition from laminar to turbulence can be promoted by plasma actuation, and that flow transition acts as an important factor in the flow-separation control. At the Begining of the actuation, each actuation can produce a spanwise vortex around the separation point near the leading edge. The spanwise vortices make the separated free-shear layer unstable and shedding away, move downstream along the upper wall, and bring outer flow with high kinetic energy into the near wall region to change the flow structures over the airfoil. Then, the flow around the leading edge of the airfoil reattach after encountering a separation bubble because of the earlier transition. At last, the flow on the upper surface of the airfoil become fully attached.