This paper numerically studies a new zero-net-mass-flux (ZNMF) coflow jet (CFJ) active control to increase the unstart margin and efficiency of axis-symmetric mixed-compression supersonic inlets without dumping the bleed flow. The simulation is done by using the in-house CFD code FASIP that solves 3D Reynolds-averaged Navier-Stokes (RANS) equations with 3rd order MUSCL scheme for shock capturing, 2nd order central differencing for the viscous terms, and the Spalart-Allmaras model for turbulence. The CFD simulation is validated with the tested NASA VDC Inlet at Mach 2.0 for its critical condition at angle of attack (AoA) of 0◦ . Good agreement between the CFD simulation and experiment at critical condition is achieved for the streamwise surface pressure distribution, total pressure profiles at different streamwise locations, total pressure recovery, and fan face distortion. The CFD simulation indicates that the baseline inlet remains started at AoA of 1.2◦ and unstarts at AoA of 1.6◦ . A CFJ inlet concept is studied to improve the baseline inlet performance. The CFJ flow control withdraws the mass flow of boundary layer at the same throat position as conventional bleed, but injects the mass flow back to the inlet diffuser area downstream. It thus has a constant mass flow from the supersonic inlet entrance to the engine entrance similar to subsonic inlets. The mechanism of CFJ active flow control to stabilize axis-symmetric inlet is two-fold: i) The suction in the throat bleed region has the same effect as conventional bleed to remove the thick boundary layer and increase flow passing capacity. ii) The downstream injection further energizes the boundary layer in the diffuser region to reduce flow blockage, increases flow passing capacity, and increases inlet stability against unstart. With a 5% less bleed flow than that of the baseline inlet, the CFJ inlet is able to stabilize the inlet at AoA of 2◦ with higher inlet stability and total pressure recovery, while keeping the constant mass flow in the inlet. More numerical studies to investigate the working mechanism of CFJ inlets are in progress.