Aiming at the issue of unreasonable particle size distribution in the overflow product from the first-stage cyclone of the primary mill at a copper mine, which led to low copper recovery in the flotation process, the optimal steel ball proportion for the grinding operation in this stage was deduced based on the nature of the ore fed into the mill and the semi-theoretical formula for ball diameter. The optimal proportion was determined to be m(Φ70 mm):m(Φ50 mm):m(Φ40 mm):m(Φ30 mm) = 15:25:35:25. Based on this steel ball proportion, the process conditions for grinding concentration and steel ball filling rate were optimized, ultimately determining the optimal grinding concentration to be 73% and the optimal steel ball filling rate to be 41%. Compared with the period before the industrial test, during the industrial test stage, the content of the selectable particle size fraction of -0.020+0.010 mm and the easily selectable particle size fraction of -0.074+0.018 mm in the overflow product increased by 10.44 percentage points and 15.18 percentage points, respectively, while the content of the over-ground particle size fraction of -0.010 mm decreased by 10.22 percentage points. Ultimately, the copper concentrate recovery rate was improved by 0.65 percentage points.