Abstract: Crack propagation behaviors of multiple site damage (MSD) in fuselage lap joint were studied based on experiments and three-dimensional finite element analysis. The crack propagation life, MSD failure life, a-N curves of MSD cracks, and the crack propagation rate were obtained from twenty specimens of three-row lap joint panels. Statistical analysis of the crack initiation life and the crack propagation life of the specimens finds that the MSD crack initiation life is negatively correlated with the crack propagation life. When individual crack initiates earlier, it is not affected by other cracks, and has a longer crack propagation life. When a large number of cracks initiate later in the entire life, they interact seriously and propagate rapidly. Based on the crack propagation rate obtained by the seven-point incremental polynomial method, it is found that the crack propagation rate changes slowly in the preceding period; when multiple cracks appear, the cracks affect each other significantly, and the crack propagation rates become unpredictable. Three-dimensional finite element analysis reveals that, as the crack length increases, the load on the subsection where the crack is located decreases, and the slow change in average stress is the reason why the crack extension rate does not increase with the crack length, and the first crack will "wait" for the appearance of the subsequent cracks, which makes the lap joint structure more prone to MSD.