Abstract: Blowing control of seamless flap based on Coanda effect can greatly increase the wing lift and improve the take-off and landing performance of transport aircrafts. Therefore, it is essential to investigate the influence of ground effect on the blowing control. In this paper, the blowing control mechanism of a seamless-flap airfoil, as well as the ground effect on aerodynamic performance with and without blowing, was analyzed by numerical simulation from the perspective of flow field variation. Flap blowing creates a local low pressure area near the Coanda surface, forming a pressure gradient pointing to the Coanda surface, and then causes the mainstream above the jet to deflect and accelerate, resulting in a clockwise velocity increment in the entire near-wall area of the wing surface. The velocity increment near the wing surface increases the pressure of the lower surface and strengthens the suction of the upper strengthens. The increase of suction on the upper surface of the main wing is the main source of airfoil lift increment. Without blowing, the ground effect reduces the flow velocity near both upper and lower airfoil surfaces, increases the pressure around the airfoil, and reduces the airfoil lift as a whole. The flap blowing with ground effect enhances the blocking effect of the lower surface on the incoming flow, and further increases the pressure of the lower surface. The flap blowing accelerates the airflow on the upper wing surface, which can compensate the velocity reduction and the suction loss of the upper surface caused by the ground effect.