To clarify the working principle and internal flow field distribution of the ultrafine horizontal graded mill, a three-dimensional model of the graded mill was established, and grid division and independence verification were carried out. Based on Fluent software, simulated the turbulent characteristics of the fluid in the graded mill, and the Euler-Lagrange approach was employed to simulate the gas-solid two-phase flow between continuous air phase and discrete particle phase. Velocity and pressure were used as evaluation indicators to analyze the interactions within the internal flow field. The results show that under the high-speed rotation of the grinding disc, the velocity and pressure on the Z=-100 mm plane of the crushing chamber exhibit a trapezoidal distribution from the center outward, with alternating characteristics along the circumferential direction of the grinding disc. The maximum velocity (up to 11 m/s) occurs between the grinding disc and the toothed grinding wall, while the highest pressure (4 200 Pa) appears ahead of the blade motion. On the Z=150 mm plane of the working chamber, the elutriation flow field and classification flow field interact, with velocities decreasing to approximately 5 m/s and pressure increasing to 4 000 Pa at the interface between the two flow fields. The crushing and classification zones achieve dynamic flow field coupling through the elutriation zone, effectively separating crushed particles and improving efficiency. This study revealed the alternating characteristics and dynamic coupling mechanisms of the internal flow field in a graded mill through multi-scale flow field coupling analysis. The findings provide significant engineering guidance for optimizing structural parameters and enhancing energy efficiency of graded mills, offering theoretical references for flow field regulation and performance improvement in similar powder processing equipment.