Abstract: While the loading outer general pipes of a certain type of engines were working, cracking or fracture occurred near the four-way seat of the oil inlet elbow for several times. In order to find out the failure cause, macroscopic analysis, fracture observation and other physical and chemical analysis were carried out to evaluate the material, welding quality and fracture characteristics. In addition, the structural features of the assembly of the pipe and the working characteristics of the afterburner of the engine were analyzed. The results show that owing to the larger rigidity of the assembly structure of the outer piper, there was excessive alternating thermal stress, which is the main cause for the fracture of the pipe. By reducing the rigidity at the maximum stress area of the pipe, the alternating thermal stress can be significantly reduced.