By using a new type of laser measuring instrument, a particle dynamics analyser (PDA), radial and axial velocity components and the size of solid particles in a hydrocyclone were measured, and the concentration distribution of the solid particles was obtained. In analysing and discussing the separation mechanism of the solid particles, as well as the main causes of the leakage of coarse particles to the overflow and the abrasion on the hydrocyclone wall, some new opinions are put forward.