Abstract: High cycle fatigue fracture occurred in a 7D68 aluminum alloy thin-walled rotary drum during the test. The microstructure and fracture mechanism of the fractured rotary drum were systematically studied by means of chemical composition analysis, hardness test, scanning electron microscopy analysis, and energy dispersive X-ray spectroscopy. The results show that there were significant differences in the microstructure and composition between the fragmented drum and the normal drum. There were abnormally enriched iron and silicon inclusions in the fragmented drum. These inclusions not only reduced the tensile strength of the material, but also became the sources of fatigue cracks under cyclic loading, which significantly accelerated the crack growth rate and eventually led to the fragmentation of the rotary drum during the test.