<p>The controllable speed casing (CSC) represents an innovative casing treatment technology specifically developed to enhance compressor flow stability. Research on the CSC has not been conducted on the low-speed multistage compressor. This paper numerically investigates the effects of CSC on the flow stability of the low-speed three-stage axial compressor (LTAC). It aims to clarify the position and cause of the first stall in LTAC and to evaluate the potential of applying the CSC to the first stall stage. For this purpose, the mechanisms by which the direction and speed of the CSC’s rotation affect the flow field are examined. The findings indicate that the stall in the LTAC is caused by a blockage at the second-stage rotor tip. In the second stage, when the CSC rotates in the same direction as the rotor, the reduction in circumferential velocity of the tip leakage flow (TLF) induces reverse flow within the passage, creating a strong blockage effect on the incoming flow and advancing stall onset. When the CSC rotates in the opposite direction to the rotor, the increased circumferential pressure gradient accelerates the TLF’s circumferential migration and suppresses the dissipation of the leakage vortex, reducing tip blockage and enhancing flow capacity. The stable operating margin of the LTAC increases with the rise in rotational speed of the CSC. When the rotational speed reaches 100% of the rotor design speed, the stable operating margin shows a maximum improvement of up to 48.67%. Under this condition, the critical position limiting the stable operation of the LTAC will transform from the second-stage to the third-stage rotor blade tip regions.</p>