Abstract The law of ball size distribution in the horizontal planetary ball mill is studied by the discrete element method. The results show that the maximum impact energy could be acquired when filling rate is 24%, moreover the biggest mean contact force and the highest energy utilization ratio of balls could be acquired when the speed ratio is 1.5. The mean contact force increases with the proportion increasing of the large balls, which means ball size distribution has some effect on the crushing and grinding process. And according to the experimental results for comparative analysis, the specific rates of breakage S i increases with the proportion rise of large balls in ball size distribution, meanwhile S i can be determined by the mean contact force( F mcf ): S i = A · 10 kf mcf , where the constant A is 0.437, the gradient k is 0.0252. At last the specific surface area of the product is measured and the particle size distribution (under 80 μm) is analyzed by Rosin–Rammler–Bennet equation. The results show that with the proportion increasing of the large balls, the specific surface area decreases, while the uniformity coefficient and the characteristic diameter increases.