To systematically elucidate the influence of the cylinder-to-cone ratio on the internal flow-field distribution and particle separation efficiency of cyclone separators, a rigorous gas–solid two-phase CFD model was developed in ANSYS Fluent. Numerical simulations were conducted for three distinct cylinder-to-cone ratios—1.33, 1.78 and 2.14—to quantify their respective effects on vortex development and particle separation behavior. Simulation results show that: the cyclone separator with a cylinder-to-cone ratio of 1.33 first forms a high-velocity flow field and a stable velocity field, its velocity distribution around the inner wall is more uniform, the cylindrical section exhibits a higher velocity, while the conical section exhibits a lower velocity, which is more conducive to the separation and discharge of larger particles. Simulation results show that cyclones with cylinder-to-cone ratios of 1.33 and 1.78 are suited for separating larger-diameter particles, whereas the cyclone with a ratio of 2.14 is more appropriate for separating smaller-diameter particles; among them, the cyclone with a cylinder-to-cone ratio of 1.33 achieves the best separation efficiency.