Optical microscopy and scanning electron microscopy were used to observe and analyse the microstructures of large hollow grinding ball residues made of low-alloy steel that cracked during use, and their hardness and impact toughness were determined. The results show that inclusions and microscopic shrinkage cavities exist in the microstructures of large low alloy steel hollow grinding balls, as well as uneven and thorough microstructure transformation, resulting in significantly lower impact toughness and higher hardness, causing cracking of the hollow grinding balls during operation. Measures to prevent cracking of large low alloy steel hollow grinding balls have also been proposed.