Abstract: The fracture accident of S30409 stainless steel wing valve of cyclone separator in a catalytic cracking unit occurred during service. The fracture reason of wing valve was analyzed by means of chemical composition analysis, metallographic examination, mechanical property test, fracture analysis and energy spectrum analysis. The results show that the wing valve was in service at high temperature for a long time, and a large amount of σ phase was precipitated in the microstructure, which led to the embrittlement of the material and the decrease of plasticity and toughness. The discontinuous shape and size of the weld caused a large stress concentration. Under the action of the gas-solid two-phase flow pressure and the thermal stress generated by the heating and cooling of the device, micro-cracks were initiated and the cracks continued to expand, eventually leading to the fracture of the wing valve.