Combining the existing slitting tip extension with the noise reduction technique proposed in this paper，based on the 30P30N multi-segment aerofoil, unsteady flow of three-dimensional airfoil structure is simulate by DDES algorithm. Firstly, the pressure and vorticity transient parameters of the model are obtained. And then, the sound pressure level and directivity are studied by FW-H integration method to study the effects of slat cusp geometric extensions on the surrounding flow field and noise characteristics. Finally, based on the instantaneous vortex structure analysis method of ,the generation and evolvement of flow structures in slat cove and corresponding noise distributions are investigated for various extensions. The investigation shows, cusp extension can effectively change the distribution of parameters of slat flow. Among which, extensions along the sheer layer can reduce the strength of turbulence kinetic energy in slat cove and decrease the characteristic dimension of vortices of separating flow, as a result, suppressing the radiation of slat noise (narrow band noise can be reduced by 3dB in low frequencies).