The vortex finder is essential in cyclone separators, significantly affecting separation performance via its diameter and insertion depth. The current study shows that as the insertion depth of the vortex finder increases, the separation efficiency initially increases and then decreases, and there exists a maximum point with which the corresponding insertion depth is the maximum efficiency insertion depth (S_MEID). However, there are inconsistent conclusions in the existing literature regarding the maximum efficiency insertion depth and a lack of explanation for the flow field mechanism at the maximum efficiency insertion depth. This study examines the Stairmand type cyclone using 13 μm silicon micro-powder, employing numerical simulation and cold mold experiments to explore the effects of the vortex finder's insertion depth and diameter on separation performance and flow field. The results indicate that the insertion depth has minimal impact on pressure drop. The maximum efficiency insertion depth of the vortex finder decreases as the diameter decreases and is independent of this insertion depth with respect to the inlet velocity. Analysis of the flow field reveals that the maximum efficiency insertion depth is essentially the result of a "competitive and synergistic" mechanism between the annular space separation capability and the separation space separation capability.