To address the impact of turbulent interference on the stability of the flow field and separation performance of hydrocyclones, we propose a design that incorporates a helical downward structure in the feed section. This design not only mitigates turbulence but also facilitates a preclassification effect. Numerical simulations were used to investigate the impact of the downward height on the internal flow field and separation performance of a hydrocyclone. An increase in the downward height reduces the static pressure, tangential velocity, and turbulence intensity of the inner flow field of the hydrocyclone while increasing the axial velocity at the inner swirl, which improves the processing capacity of the hydrocyclone. In addition, this increase in the downward height reduces the degree of particle mismatch and improves the classification accuracy. Compared with the conventional tangential inlet hydrocyclone, the optimized helical downward hydrocyclone achieves higher separation accuracy, demonstrating distinct advantages.