Abstract: In order to improve the service life and service quality of 42CrMo4 steel for connecting rod of a diesel engine, the ultrasonic shot peening process was used to treat the 42CrMo4 martensitic steel. The effects of power, pin diameter and tool speed on surface integrity such as surface roughness and surface hardness were studied. Then, according to the relevant national standards, the ultrasonic shot peening strengthened fatigue test piece was designed, and the high cycle fatigue test of the test piece treated by the ultrasonic shot peening strengthening process was carried out, and the experimental results were analyzed with the traditional shot peening results. Finally, the fatigue fracture was characterized by scanning electron microscopy, the evolution of microstructure was observed, and the fatigue damage failure mechanism was revealed.