Abstract: In this paper,The reason that causing the fracture of bracket triangle plate of GIS angle disconnector in a 220kV substation was investigated by macroscopic observation, material composition test, mechanical performance test, microstructure observation and energy spectrum analysis. The results show that, the improper heat treatment of raw materials lead to deterioration of microstructure and decrease of tensile strength of bracket triangle plate, and local segregation defects exist in the stress concentration position of bracket triangle plate, in addition, the installation of bracket triangle plate，pull rod, moving and static contact terminal does not meet the requirements, which makes the stress of bracket triangle plate increase, then occurring fracture from the stress concentration position when separating brake。