The cross section morphology and metallographic structure in the weld zone of the 2024-T351 aluminum alloy friction stir lap welded joint was observed by experiment.It indictated that there are asymmetric hook-shaped defects on both the advancing side and the retreating side of the friction stir lap welded joint.Friction stir lap welded joints were tested under constant-amplitude fatigue loading on MTS test frame to obtain the S-N curve of nominal stress amplitude.The elastic-plastic finite element model of lap joints was established according to the test.Using the SWT fatigue damage equation and cyclic stress strain data of stress concentration region from finite element analysis,the fatigue lives of lap joints were predicted,and further compared with the experimental results. The results show that the prediction results for the low-cycle fatigue lifetime of friction stir lap joints by the SWT fatigue damage formula are close to the experimental results,and the errors are all within 2 factors,while large error is in the prediction results for the high-cycle fatigue lifetime.At the same time,it shows that the stress concentration of lap joints has greater influence on the fatigue lifetime than the effective thickness.