Abstract: During the routine inspection of the main nozzle of a helicopter engine, cracks were found in many parts of the main nozzle. The cracks were mainly located at the reinforced edge and the edge of the screw hole on the flange edge of the main nozzle. The cracking causes of the main nozzle were analyzed by means of macro observation, macro analysis of fracture, micro analysis of fracture, metallographic examination, observation the characteristics around the crack source at flange edge, flatness verification of the flange edge and welding quality test of the reinforced edge. The results show that the cracks on the main nozzle were fatigue cracks. The flange edge cracks originated from the extrusion trace on the front of the screw hole. Due to the poor service environment (high temperature and air flow vibration), the single-sided gasket of the flange edge connection holes could not bear the vibration load and formed the fatigue cracks. The cracks in the reinforced edge originated from the fusion point of the end butt joint. The resistance spot welding and fusion welding coexist in this part, and the material properties of the heat affected zone decreased. The butt weld at the end of the reinforced edge was located in the long side with the largest vibration, and the fatigue cracks were finally formed.