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The influences of axial-slot casing treatment (ASCT) on the performance and unsteady tip region flow are experimentally and numerically investigated in a highly-loaded mixed-flow compressor. The total pressure ratio, stall margin, and efficiency of the compressor are improved with ASCT. Static wall pressure was measured using unsteady pressure taps installed on the casing to identify the stall inception and resolve the tip region flow. The compressor stalls through spike-stall inception with the spillage of tip leakage flow (TLF). FFT analysis of dynamic pressure data shows that unsteady tip clearance flow (TCF) characterized by a frequency band of 0.4–0.6 blade passing frequency exists for both cases with and without ASCT. The addition of ASCT alleviates fluctuating amplitude of TCF in the blade passage but enhances oscillating strength of the main flow located upstream of blade leading-edge. Time-averaged solutions of unsteady simulations indicate that ASCT suppresses the spillage of TLF through suction and injection effects. However, flow separation located downstream of slots is enlarged by ASCT, which is attributed to the increased blade tip load. To reveal the underlying mechanisms, the tip flow field was further studied at various time instants. The fluids with high enthalpy are injected from slots and then divided into two parts by the rotor blade. One part of the fluids attaches on blade pressure side causing the local high static pressure spot, which leads to the increment of pressure loading at blade tip. The other part interacts with TLF periodically, thus pushing it into passage and delaying rotating stall.