Abstract: By using RSM (Reynolds stress model) turbulence model and VOF (volume of fluid) multi-phase flow model, systematic numerical studies on the influence of vortex finder diameter on steady flow field of Φ50 mm hydrocyclone were investigated. The flow characteristics were analyzed in terms of air core, LZVV, short circuit flow and turbulence intensity. Further the range of vortex finder diameter was determined for steady flow field of hydrocyclone. Based on the physical experiments verification, the steady flow field optimized improved the separation efficiency. Results show that when the vortex finder diameter is too small, the air core cannot form completely, the turbulence intensity in the separation space is higher, the flow divertion ratio is large and the short circuit flow is smaller. With the increasing of the vortex finder diameter, the air core is gradually developing, the turbulence intensity reduces, the symmetry of LZVV is gradually enhancing and the flow divertion ratio is gradually reducing. Thus, the flow field stability and the separation performance are enhanced. With further increasing of the vortex finder diameter, the air core diameter and the short circuit flow increase, the stability of the flow field decreases inversely, consequently, the separation efficiency decreases. Therefore, a compromised optimum vortex finder diameter for the Φ50 mm hydrocyclone is around 0.30 D.