Abstract: With the special demand of STOL performance for transport plane, the regularly mechanical devices for lift enhancement could no longer satisfy the requirement of maximum available lift coefficient. The internally blown flap system is the most efficient way of powered lift-enhancement for fixed-wing aircraft. In order to promote the engineering application of this technology, the aerodynamic characteristics and flow structure of a 2D subsonic 60° seamless flap with varying moment coefficient are obtained based on RANS equations. The mechanism of circulation control and stall in different regime have been analysed. It reveals that, the lift-enhancement efficiency (the ratio between the gain in lift yielded by the active system and the required blowing momentum) of this system is high, which reaches the highest value of 70 at the critical momentum, and the lift coefficient increase by 125% compared to that of without blowing. With blowing, the lift coefficient increases due to the circulation increases on the main wing surface, the increment is about 78% of the total lift coefficient increment. The location of aerodynamic center of the airfoil moves backward with the increase of blowing flux. Further, blowing delays flow separation at the boundary-layer control regime, resulting in a significant lift coefficient improvement. The increment of lift coefficient is achieved by a further defection of the streamlines due to the jet effect downstream of the trailing edge at the super-circulation regime. With the blowing flux increase, the stall angle decrease in the boundary layer regime and increase at the super-circulation regime.