Abstract: In line with the principle on which classification is affected in cyclone,and by applying the computational fluid mechanical principle and method,numerical simulation of the flow field inside the cyclone is made at varying cyclone apex diameters and properties of feed material.Through the simulation,the effect of the above-going 2 factors on classification performance of the cyclone has been brought to light as evidenced by the following findings: with the increase of apex diameter,the flow velocity decreases,and the zero-velocity envelope tends to shrink both inwardly and upwardly,causing eventually the diminution of classification size; and,with the increase of density or size or mass concentration of feed,there can be seen a drop of the radial velocity of solid particles toward the center,along with a higher yield of coarser particles and higher content of finer size material in underflow.Therefore,to have the structural parameters of a hydrocyclone rationally designed according to specific operating conditions on site is of vital importance especially for the control of classification size and improvement of classification performance.