Casing treatment is one of the important solutions to enhancing compressor stability. A new concept of controllable speed casing was proposed in this paper, aimed at weakening the loss of efficiency and achieving off-design regulation. In the context of controllable speed casing, the whole casing structure was divided into stationary ring section and rotatable ring section. With adjustment made to the rotating direction and speed of the rotatable ring, different additional effects could be exerted on the tip flow field. In the paper, a numerical research was carried out on the flow stability of different rotating directions and speeds of the rotatable ring. The results show that the controllable speed casing rotating in whether same or opposite direction of the rotor can improve the stability margin of the rotor by controlling the interaction between shock wave/leakage vortex. Moreover, the controllable speed casing with different rotating speeds produces different stable operating margin improvements. When the casing is in the same rotating direction and speed as the rotor, the improvement of the flow field is the most obvious, and the stable operating margin could be increased by 52.56% on the premise the efficiency is basically unchanged.