This paper proposes two kinds of structure-modified hydrocyclone that exhibit better performance than the traditional ones. In type-1, in which the outer edge of the vortex finder is hexagonal, the separation efficiency increases by 18.1 % compared to the conventional hydrocyclone, while type-3, with a hexagonal conical section, can save 49.7 % of energy. To determine the reasons for the improvement in performance, numerical simulations are conducted to observe the flow field inside the hydrocyclones. Tangential, radial, and axial velocities are all studied in this research according to the simulation results. The radial velocity has a close relationship with separation efficiency. Based on the analysis, the separation mechanisms of different types of hydrocyclones are proposed. Furthermore, the air core, which is captured by a CCD camera, as a factor correlative to the flow pattern is also supportive of the improvement. The smoke area, which has many small air bubbles around the vortex finder, is defined in this work. The existence of the smoke area has a negative influence on energy transfer and separation efficiency.