Abstract: In order to understand the reason of the fracture of the piston rod used in aircraft landing gear during the service process, its microscopic morphology was investigated using SEM, and the root causes were studied. The results showed that the fatigue fracture under cyclic stress during the service process leads to the formation of micro-cracks on the surface of the piston rod, and the formation of micro-pores in its interior, where dislocations gathered and stress concentrated. With the increase of the cyclic times of stress, the number and size of micropores increase continuously, which, finally, leads to the fatigue fracture of the material. By means of optimizing the heat treatment, the strength and toughness of the connector can be well balanced, and its fatigue resistance can be improved in practice.