Abstract: In order to improve the cruising performance of long-endurance UAVs (Unmanned Aerial Vehicles), experimental studies on separation flow control over a two-element airfoil using a symmetrical DBD (Dielectric Barrier Discharge) plasma actuator which was mounted at the leading edge of airfoil at low Reynolds number were conducted in a low-speed wind tunnel using force measurement and two-dimensional PIV (Particle Image Velocimetry). The influences of Reynolds number (Re) on control effect were analyzed, and the effects of increasing circulation and suppressing separation in control were discussed. During the experiment, the angle of attack of airfoil was fixed at zero, with Re ranging from 4.14 × 104 to 1.62 × 105. The results indicated that the relationship between the control effect of plasma actuator and Re is nonlinear. As Re increases, the control effect first strengthens and then weakens. The best control performance occurred at moderate Re values (1.05×105 to 1.21×105) and the drag was reduced slightly (approximately 1.3%) and lift was increased dramatically (approximately 20.68%) with plasma actuation, thereby improving airfoil aerodynamic performance. The implementation of flow control of plasma actuator is mainly achieved by suppressing separation flow and increasing circulation, with different proportions of the two effects under different Re conditions. At lower Re, the plasma actuator promoted the instability of the separated shear layer and induced a series of spanwise vortices which were closer to the wall surface. Therefore, the effect of suppressing flow separation was dominant at lower Re. Under moderate Re, the plasma actuator can be capable of increasing the flow velocity around the upper surface of the airfoil and produced a train of small-scale spanwise vortices near the separated shear layer. Then, these small-scale vortices were merged into a large-scale spanwise vortex, thereby suppressing the flow separation.