The unsteady numerical investigation of three kinds of self-recirculating casing treatments was performed to explore the mechanism of affecting the ability of improving compressor stall margin with changing axial position of casing treatment(CT). Results showed that the ability of improving stall margin was best when the injecting part of CT was close to the blade tip leading edge

and also located in upstream blade inlet. The ability became low by changing the axial position of CT to make the injecting part above the blade tip leading edge. It became lower by moving the injecting part away from the blade tip leading edge along upstream direction

and the corresponding stall margin improvement gained with three kinds of casing treatments was 9.40%

7.09% and 5.49% respectively. When the injecting part of CT was close to the blade tip leading edge

and located in upstream blade inlet

the level of positive effect created by CT was highest in the blade tip leading edge and the stall of rotor happened in this pivotal zone. Consequently the inlet blockage caused by the low energy tip clearance leakage flow was restrained most effectively

and the corresponding ability of increasing rotor stable operating range was highest among three kinds of casing treatments.