Driven by the demand for purifying indoor air from particulate matter, this study has carried out systematic numerical research on the effect of cyclone separator geometry on the performance of PM_2.5 filtration. A comprehensive CFD analysis system is constructed based on three cone segments generatrices: straight, concave, and convex. The continuous phase in the simulation adopts the Reynolds Stress Model (RSM) due to its high ability to predict the strong anisotropic turbulent flow inside the cyclone separator. The particle phase in the present study is simulated through the Discrete Phase Model (DPM) by the Lagrangian method and considers the effects of drag force, centrifugal force, and turbulence diffusion to predict the migration and deposition process of particles with various diameters. From the results, the convex generatrix shows great superiority in capturing fine particles, obtaining a high efficiency of PM_2.5 separation up to 82.4% and a small cut-off diameter down to 2.1 μm. In comparison, the concave generatrix outperforms in reducing the pressure drop and optimizing the energy efficiency ratio, while the straight generatrix ranks in between. The results show that the reasonable control of geometric curvature can significantly improve the flow field stability and particle distribution characteristics.