Abstract: Ultrasonic fatigue testing is a practical method for obtaining very high cycle fatigue (VHCF) strength of welded joints. However, there is no effective method to predict and evaluate the ultrasonic VHCF life of arbitrary welded joint forms. A transient structural stress method for assessing the fatigue life of welded joints under ultrasonic harmonic resonance conditions as well as a method for fitting the master S-N curve for VHCF are proposed. Based on the published test data of 300 welded joints, a 20 kHz VHCF master S-N curve of aluminum alloy welded joints was fitted. The response equivalent structural stress spectrum was obtained by resonance model transient simulation analysis and post rain flow counting. With the spectrum and the VHCF master S-N curve, the fatigue life of welded structures is predicted. The results show that the structural stress concentration factor does not change due to vibration or external load magnitude changes in welded joints. The fitted ultrasonic VHCF master S-N curve is a new narrow band with m = 16.5, which extends the applicability of the master S-N curve method theory to the very high cycle region and can be used for ultrasonic vibration VHCF life prediction of welded structural parts with any joint forms.