Shear stress provided by a hydrocyclone was employed to remove the oil from oil-contaminated catalysts. Understanding the deoiling mechanism and quantitative analysis of the interaction between shear stress fields and deoiling are necessary to improve deoiling efficiency. A numerical simulation was conducted for the velocity field and shear stress field of a hydrocyclone. The shear stress field in the wall layer, where oil-contaminated catalysts are usually located, was robust. Increasing inlet flow rates resulted in a higher shear rate distribution along the wall layer. Numerical results were compared with experimental data. In the deoiling process, higher shear stress rates promoted faster transport of oil from catalysts into the fluid, thereby increasing the deoiling efficiency. Deoiling by the shear stress of a hydrocyclone is an efficient method for cleaning oil-contaminated catalysts within a short time.