Abstract: The fatigue life of Q345B steel cross joint was treated by ultrasonic impact treatment, and the fatigue life of the welded joint under two kinds of amplitudes was compared and analyzed. The results showed that the fatigue life of welded joints at 25 micron amplitude is 4.3~7.6 times higher than that at 18 micron amplitude. The reasons for the difference of ultrasonic impact fatigue life extension under different amplitudes were compared and analyzed in terms of macro-morphology, micro-fracture and surface structure of impact area after ultrasonic impact treatment, and the finite element model was established to simulate the stress field of welding temperature field and stress field. By comprehensively comparing and analyzing the test results with the finite element calculation results, the results showed that the greater the amplitude, the deeper the compressive stress layer and the greater the surface compressive stress of the specimen under the same impact condition.