Abstract: Aiming at the problem of dynamic stall of helicopter rotors under variable free stream during forward flight, a dynamic stall control method of airfoils based on the co-flow jet (CFJ) is developed. Taking NACA0012 airfoil as the research object, the unsteady Reynolds-averaged Navier-Stokes equations are solved based on the transition SST turbulence model, and numerical simulations of the dynamic stall of airfoils controlled by CFJ under different parameters are carried out. The results show that CFJ can effectively suppress the dynamic stall of airfoils. Under the condition of variable free stream, the jet channel has a negative impact on the original aerodynamic characteristics of the airfoil, and the power coefficient increases much faster than the jet momentum coefficient. There is an optimal working condition for CFJ to achieve the best control effect. CFJ accelerates the evolution of multi-vortex by mixing with the mainstream to suppress the dynamic stall, and enhances the kinetic energy of the chordwise flow to overcome the adverse pressure gradient to suppress flow separation and promote flow reattachment. Under the condition of Mach number 0.283, reducing frequency 0.151 and advance ratio 0.25, CFJ can increase the lift of the airfoil, reduce the drag and the negative pitching moment peak, and advance the reattachment of the flow. The overall aerodynamic characteristics of the airfoil are obviously improved by the CFJ control.