Abstract: In order to investigate the active flow control effect of synthetic jet on preventing flow separation and delaying stall of airfoil at low speed, wind-tunnel tests of jet control on the stall characteristics of NACA0021 airfoil are conducted. By systematic comparison tests including measurements about model aerodynamic forces, flow velocity on the airfoil upper surface based on Particle Image Velocimetry (PIV) technology and velocity profiles in boundary layer, the influences of synthetic jet parameters on control effects of airfoil stall characteristics are further explored. A large number of experimental results indicate that control effects on airfoil lift and stall angle of attack (AoA) due to jet angle are sensitive to the momentum coeffcient of synthetic jet. Overall, the control effects of the almost tangential jet are more effective when the momentum coeffcient of synthetic jet is large enough: increments about 23.6% of maximum lift coeffcient and 5° of stall-incidence of airfoil are obtained when the jet angle is 30° with jet momentum coeffcient being 0.033. On the other hand, a larger jet angle is required to achieve the best control effect when the momentum coeffcient is small: when the momentum coeffcient is about 0.0026, the normal jet is most effective on increasing the maximum lift coeffcient of airfoil by an increment about 9.2%.