For the grinding ball forging process with square steel blank, the characteristics of forgings and the design principles of mold cavity in the two processes of forging round bar and forging ball were explained. In the process of forging round bar, the process characteristics of forging round bar, the material flow law, the shape characteristics of forgings and the equivalent stress distribution and failure position of mold were analyzed. Then, two key design principles of mold cavity for forging round bar were summarized by comparing the forging shapes of normal circular cavity and ellipsoidal cavity, and the corresponding size range of mold cavity was given. Furthermore, in the process of forging ball with round bar, the process characteristics, the material flow law, the forging shape characteristics and the equivalent stress distribution and failure position of mold were analyzed, two key design principles of mold cavity for forging ball were summarized, and the design scope of mold cavity size for forging ball was given. In addition, the principle that the cross section and volume of special-shaped blank must be matched with the corresponding mold cavity sizes was emphasized, and the forging states that the forging sizes did not match with the cavity were enumerated. Finally, some problems that need to be paid attention to in the use of mold and blank were added, and the principle of mold design was applied to manual operation and automatic production line.