Abstract: The effect and mechanism of lift-enhancement of the co-flow jet technology was investigated by numerically simulating the flow around a co-flow jet airfoil. The Farve-averaged Navier-Stokes equation of unsteady compressible flow and the S-A turbulence model were solved by Fluent software. The results show that the co-flow jet technology remarkably reduces the zero-lift angle of attack of the airfoil, and increases the maximum lift coefficient and the stall angle of attack. The mechanism of lift-enhancement of the co-flow jet technology is analyzed theoretically. Firstly, it increases the circulation around the airfoil by accelerating the speed of the flow on its upper surface. Secondly, it decays the flow separation at the airfoil tail at high angles of attack through the injection of energy inside the boundary layer.