Abstract: The suspension porcelain insulator of a 500 kV substation broke during operation. The fracture reasons of the suspension porcelain insulator were analyzed by means of macroscopic observation, penetration test, scanning electron microscope and energy spectrum analysis. The results show that the porcelain body of the suspension porcelain insulator had the defects of inclusions and cracks, the gluing part of the porcelain part and the iron cap was cracked. Operating in long-term outdoor environment offected by external factors such as environment and load, the water molecules entered the porcelain body through the glue cracking part, causing the porcelain body to heat. The thermal effect inside the porcelain part induced the cracks to accelerate the expansion and the strength of the porcelain part was significantly reduced. Finally, the porcelain insulator broke at the neck where the stress was most concentrated.