Abstract: The L-type junction board of the 500 kV side capacitor voltage transformer (CVT) accessory of shunt rector in a substation broke suddenly in a typhoon day. The fracture causes were analyzed by macro-morphology observation, chemical composition analysis, mechanical property test, microstructure observation and finite element simulation. The results show that the fracture property of the L-type junction board was brittle fracture.The large flake or lath eutectic silicon and the AlFeSi second phase in the microstructure of the junction board were the main causes of its fracture, and the unreasonable structure design was the other cause of its fracture. Under the tensile stress and torque caused by the violent movement of the leads in the typhoon environment, the microcracks initiated at the coarse AlFeSi second phase particles in the stress concentration location of the contact part between the bolt hole of the junction board and the edge of the convex platform at the CVT top, and then extended along the brittle eutectic silicon phase, eventually leading to brittle fracture of the junction board.