Abstract: The cast ZA33 zinc-aluminum alloy worm gear in a 110 kV substation fractured during equipment operation. The fracture causes of the worm gear were analyzed by means of macro observation, chemical composition analysis, mechanical property testing, metallographic examination, fracture and energy spectrum analysis. The results show that there were casting defects such as coarse dendritic structure and porosity in the worm wheel, and the buffer rubber blanket of the mechanism was severely hardened. It caused cracks to occur at the pin shaft hole of the worm gear, and the cracks expanded during equipment operation, causing the worm gear to break. Improving the casting process of worm wheel to eliminate casting defects and replacing the buffer structure with disc spring buffer structure could prolong the service life of worm gear.