Abstract: In the construction of shale gas fracturing, the union's wing nut of high pressure pipe cracked during the service. Samples were taken from the fracture surface of the union wing nut at the failure site for macro and micro observation, fracture energy spectrum analysis, hardness testing, and metallographic examination. ABAQUS software was used to perform mechanical analysis on the union wing nut and to reveal its failure mechanism. The results show that there were multiple sources of fatigue on the inner diameter of the nut, and there were obvious bainitic lines in the center of the fracture surface, which were covered with red brown (Fe₂O₃) and yellow green (FeCl₂) corrosion products. Spherical oxides were on the surface of the micro fracture, and a certain amount of Ca and Si elements were detected by energy spectrum analysis. There were many non-metallic oxides of class C and class D in the nut raw material. The hardness and metallographic structure met the process requirements. The location of stress concentration obtained from finite element analysis was consistent with the actual fracture location, which was close to the top of the union thread, and the maximum stress value was 910 MPa, exceeding the material yield strength. The failure mechanism of the union wing nut was corrosion fatigue fracture, and the corresponding improvement measures were proposed.