Abstract: Active flow control on a three-dimensional NACA633-421 airfoil has been investiga-ted experimentally using beveled-slit-synthetic-j et-actuator (BSSJA).Balance force measurement results show that the synthetic jet can control the flow separation effectively,improve the maxi-mum lift coefficient by 10.4% and increase the stall angle of attack by 4°.Both boundary layer velocity measurement and particle image velocimetry (PIV)technology have been performed un-der selected experimental conditions to investigate the mechanism of synthetic jet flow control. Results reveal that the shape factor of boundary layer velocity profile decreases with BSSJA con-trol and the promotion of the power of the boundary layer is responsible for the control effect. The instantaneous flow maps and ensemble-averaged PIV results reveal that synthetic j et tends to enhance the flow dilution and transfer the mass flux so that the separated flow could attach to the wall of NACA633-421 airfoil again.In addition,the turbulent kinetic energy (TKE)and Reyn-olds stress are increased near the wall compared to the situation without control.