Abstract: In order to improve the fatigue performance of Ti alloy welded joints,the weld toe in cross-shaped joint was processed by the HJ-Ⅱ-type ultrasonic peening treat plant developed by authors.The test results indicate that when the stress ratio R is equal to 0.5 in loading condition,the conventional ultrasonic peening processing makes the fatigue strength of the cross-shaped joint of TC4 titanium alloy be enhanced by about 73.5%,the life be lengthened about 12-17 times,while loading ultrasonic peening processing makes the fatigue strength of the cross shaped joint of TC4 titanium alloy be enhanced by about 148.1%,and the life be lengthened about 23-26 times.Meanwhile,the mechanism for the fatigue performance of welded joints enhanced by the ultrasonic peening,especially the loading ultrasonic peening is expounded from the change of the stress concentration factor after the ultrasonic peening,the change of the residual stress of welded joints,the plastic deformation in the surface layer of the weld toe area and the change in hardness.