The influence of Axial-Slot Casing Treatment (ASCT) on the performance and stability enhancement mechanisms of ASCT were experimentally and numerically investigated in a high-speed mixed-flow compressor under three different tip clearances. Unsteady simulations showed the compressor stalled through end-wall stall route, i.e. the spike stall inception originating from rotor tip region, which was validated by dynamical measurements. When the ASCT was applied, greater than 20% of Stall Margin Improvement (SMI) could be achieved for the compressor under each tip clearance size. The streamwise velocity contours and flow structures in the tip region and axial slots were deeply analyzed to explore how the so called & ldquo;suction and injection effects & rdquo; gener-ated by the ASCT manipulate tip clearance flow and enhance the stability of compressor under dif-ferent tip clearances. It was found that the dominant stability enhancement mechanisms of ASCT varies with tip clearance size for the mixed-flow compressor.