Tailings produced by the novel wet beneficiation process for low-grade bauxite exhibit complex morphological characteristics and intricate occurrence states, posing significant challenges for efficient aluminum concentrate recovery. These complexities result in reduced recovery rates and lower concentrate grades. To address this issue, this study systematically investigates the effects of underflow orifice diameter (4, 6, 8, 10, 12 mm) on the internal flow dynamics and separation efficiency of a hydrocyclone. A pilot-scale ?75 mm hydrocyclone system was employed, with the underflow orifice diameter as the sole variable while maintaining other structural and operational parameters constant. Computational fluid dynamics (CFD) simulations, combined with experimental validation, were used to explore how variations in underflow orifice diameter influence turbulent kinetic energy (TKE) distribution and flow field stability, ultimately affecting the efficiency of tailings-aluminum concentrate separation. Simulation results indicated that adjusting the underflow orifice diameter significantly regulated the magnitude and spatial distribution of TKE, which in turn governed the evolution of short-circuit flow and recirculation flow within the hydrocyclone. An optimal balance was observed at underflow orifice diameter of 8 mm, which effectively stabilized the internal flow field, controlled short-circuit and recirculation flow, and enhanced the dissociation of tailings from aluminum concentrate. Experimental results, conducted under feed pressure of 0.3 MPa, feed concentration of 17.15wt%, and an alumina-to-silica mass ratio (A/S) of 2.74, confirmed that the optimized 8 mm underflow orifice hydrocyclone significantly improved separation performance. Compared to the baseline design, the 8 mm orifice increased underflow concentrate yield by 9.33% and improved the A/S ratio by 20.39%, effectively mitigating coarse particle overflow and fine particle entrainment. Strong agreement between CFD simulations and experimental data validated the reliability of the numerical approach. The findings provide a theoretical framework for optimizing hydrocyclone designs in complex slurry separation processes, particularly for low-grade bauxite beneficiation. By elucidating the interplay between TKE modulation and flow structure evolution, this study offers valuable insights for industrial applications, contributing to the development of more efficient hydrocyclone separation strategies.