The thickness of the overflow pipe in hydrocyclones significantly affects performance indicators, including production capacity, particle size separation, and separation efficiency. This paper examines the impact of the relative outer diameter of the overflow pipe on the separation performance of two types of hydrocyclones, graded and thickened, using numerical simulation and physical experiments. The study reveals varying effects on the two types of hydrocyclones. The findings indicate that the overflow pipe wall thickness for the classifying hydrocyclone should be kept to a minimum while maintaining mechanical strength, and that the thickening type hydrocyclone should choose an ideal overflow pipe thickness. In this study, the thickening hydrocyclone achieved the optimum thickening results when the relative outside diameter of the overflow pipe was 0.72, resulting in a 20% reduction in pressure loss over the minimal relative outside diameter of the overflow pipe. The results of this study provide an important reference for the structural optimization of hydrocyclone design.