Steady and unsteady simulation is conducted on a transonic compressor with semi-circular slot casing treatment. The axial momentum in the rotor tip and at slot openings is also calculated. The interaction between the rotor tip slot openings is analyzed from the perspective of momentum transportation. Time-averaged results of the blade tips show that casing treatments suppress inverse momentum, which could improve the stall margin. The higher overlap leads to a result that the minimum of axial momentum distribution at slot opening moves downstream. But the minimum location relative to the rear of slots is still unchanged. Finally, the change of momentum distribution at the slot opening is elucidated by integrating the transient result of axial momentum at slot opening and the flow field at rotor tip region.