Abstract: In response to the problems of hard rock accumulation and limited production capacity improvement in a large molybdenum mine using a semi−autogenous mill, the modified Azzaroni formula was used to accurately calculate the optimal steel ball diameter. Various crushing and grinding parameters obtained through different testing methods and compression experiments to measure hard rock particle size were incorporated into this calculation. The rationality was verified through laboratory experiments. The research results indicated that: (1) The crushing and grinding parameters obtained by the three testing methods consistently highlighted the difficult grinding and stripping characteristics of the ore, though variations in describing the impact resistance of the ore were observed; (2) The hard rock particle size of the ore was determined to be −80+20 mm; (3) The optimal steel ball scheme for a semi−autogenous mill was a 1∶1 mass ratio of 130 mm and 110 mm diameters. Using a single ball diameter of 130 mm with a 100 mm diameter as the self−grinding medium resulted in an 11.17% increase in production compared to the existing 120 mm ball diameter scheme in the selection plant, and a 1.67% decrease in the content of hard rock −80+20 mm. The content of+100 mm with a mixed ball diameter of 130 mm and 110 mm in a 1∶1 ratio increased by 1% compared to a single ball diameter of 130 mm, while the content of hard rock −80+20 mm decreased by 3.41%. This verified the accuracy of the calculated maximum steel ball diameter and achieved better grinding results using the mixed ball diameter scheme.