Abstract: Fine particles have become the bottleneck of "poor, fine, and miscellaneous" minerals recovery due to slow settling speed, low separation efficiency, and small processing capacity per unit area. To significantly improve the recovery rate and separation efficiency of fine coal particles, optimal separation process parameters were explored. By analyzing the complex motion characteristics of particles in the inclined channels of the Reflux Classifier and their trajectories upon entering the inclined channels, a solid foundation was laid for establishing an accurate theoretical model. The critical condition particle separation theoretical model established in this manuscript takes into account multiple key influencing factors, such as particle properties, equipment parameters, terminal velocity of particles, and Reynolds number, providing a theoretical basis for achieving efficient separation of fine coal particles. Additionally, the accuracy of this model was verified through particle motion equilibrium tests and fine coal separation experiments. The results indicate that the theoretical model prediction results are highly consistent with the actual separation results under the 6 mm channel spacing, and demonstrate optimal separation performance. When the channel spacing is too wide, the size differences between particles significantly affect the separation, leading to a decline in separation efficiency.