Abstract: High speed centrifugal compressors have been increasingly used in small aero-engines and gas turbine industries.However,knowledge of the 3D viscous flow in this kind of impeller flow passage is quite limited.Based on the CFD results validated previously with experimental data,this paper analyzed and discussed in detail the 3D flow within a high-speed centrifugal impeller at near-design condition in order to make deep understanding of the features of shock/boundary layer interaction and its secondary flow pattern.