Abstract: A numerical investigation into the suppression of slat noise has been conducted with upstream parallel mass injection near the slat cusp. The efficiency and mechanism of noise attenuation with upstream parallel mass injection is studied in a hybrid approach. Delayed Detached Eddy Simulation is performed to describe the noise sources of a two component high lift model, FREQUENZ; Ffowcs Williams and Hawking’s integration is employed to obtain the far-field aeroacoustic signatures. Results show that with injection, aerodynamic capability of the high lift devices is sustained, and both broad band and tonal noise attenuation are observed. With detailed analysis of the time-averaged and the instantaneous flow fields, two mechanisms are identified as the reasons accounting for the noise attenuation: (1) the interaction of upstream parallel mass injection and the detached flow at the cusp alters the characteristics of the free shear layer, leading to the breakup of the large coherent structures and creation of smaller vortical structures, these reduced scale vortical structures are well confined in a narrow space, thus reduces the shear layer flapping and leades to a smaller impingement between free shear layer and slat pressure surface, and then eventually reduces the fluctuating pressure levels; (2) upstream parallel mass injection reduces the velocity of the back flow, which alleviates the feed back loop.