Abstract: A CFD-based numerical simulation of the flow field inside a hydrocyclone was carried out at different levels to reveal the influence of apex diameter and cone angle on the flow field. The results showed that the flow velocity of the field decreases, with increasing apex diameter, then the enveloping surface at zero axial velocity shrinks. When the cone angle increases the size of the enveloping surface changes, and the smaller the cone angle, the less the influnce of apex diameter on the velocity field within the enveloping surface and the more unobvious the effect of improving separation. With the apex diameter increased, the pressure of the flow field decreases but the pressure gradient within the apex region increases differently in accordance to the change in cone angle. The phenomenon reveals the interactive influence between the cone angle and apex diameter in the flow field inside a hydrocyclone to a certain extent.