A case with circumferential grooves was designed for a transonic compressor.And 3-D numerical simulations were carried out for stall mechanism in three operative modes at representative speed.Comparison study between compressors with and without casing treatment shows that: with the rotation speed rising

stall margin gain increases dramatically under the help of casing treatments;and the case with middle grooves has better compromise between stall margin and efficiency.Under low speed

the gain reduces

and the last grooves on the chord of blade tip have greater influence on the stall boundary.Further investigation shows that there is a translation in the mechanism of compressor stall when its speed reduces: the stall margin gain mainly results from suppression of tip leakage vortex by casing treatments under high rotation speed;whereas under low rotation speed

the gain benefits more from reduction of the boundary layer separation on suction side of blade tip.