Numerical simulation of stage 35 with bulk swirl is carried out using CFD technology for the convenience of analyzing the instability mechanism of transonic compressor. The key factors triggered instability of the compressor are discussed such as leakage vortex and radial vortex at different rotation speed and swirl distortion condition. The results show that positive bulk swirl restrains flow separation of suction side, the leakage vortex, volume of radial vortex of compressor are decreased in positive bulk swirl. The flow separation, radial and leakage vortex and blocking area are increased with low rotation speed in negative bulk swirl. At high rotation speed, negative bulk swirl has little influence about leakage vortex, but the volume of radial vortex is greatly increased to drive flow gathered on blade tip to cause large low speed area in the tail of suction side, causing the instability of compressor strictly.