The mechanism and efficiency of noise control in supersonic cavity flows with steady upstream mass blowing are numerically investigated. A slotted jet is placed in the upside of cavity leading edge. The mass blowing is simulated by specifying a vertical velocity ejecting through the slotted jet. The steady upstream mass blowing is an effective approach for the noise suppression in supersonic cavity flows. The strength of the resonant noise and the broadband noise are decreased with a delightful amplitude, that is, approximately 15 dB SPL decrease in the dominant mode and 5 dB SPL decrease in the broadband noise. Two primary mechanisms are addressed for the noise control with steady upstream mass blowing, lifting up of the cavity shear-layer and disruption of shear-layer instability.