Abstract: The field in tip clearance of some compressor stator was simulated by Numeca CFD, as well the field in casing treatment slots. The results were compared and analyzed, the computation showed detail streamline in tip clearance and slots. By the casing treatment, the flow path was changed, the tip leakage eddy can be passed through, so the blockage of low incline streamline can be moderated. The field near the tip enterd into the inclined slots by the high pressure from the tailing pressure side, then jet into field near the leading suction side. The jet fluid sweeped off the stalled flow, so the flow separated was postponed, and the stable operating flow range was increased, and the quadratic flow loss was decreased. The high jet effect on main flow from momentum exchange may be primary reason that slot casing improved the stable operating range.