Abstract: The microscopic damage evolution of 2A12-T4 and 2A12-O aluminum alloy friction stir welding during tensile deformation was observed by optical in-situ tensile tests, and the crack growth path of the mismatched joints was analyzed. The results show that the fracture of the 2A12-O high-matched joint in the tensile test is in the base material, while that of the 2A12-T4 low-matched joint is near the HAZ zone.The second phase particles of high-matched joints and low-matched joints are not matched with the grain properties of the base material, which leads to the fact that the cracks of low-matched joint are mostly generated from the second phase particles, and the cracking degree of the larger second-phase particles is obviously greater than that of the smaller ones and matrix, while the cracks of high-matched joint are mostly generated inside the grains. The crack propagation paths of both the high-matched and the low-matched joints are mainly transgranular propagation. The microcrack, large-angle grain boundary and second-phase particles in the high-matched joints hinder the crack propagation, while the broken of second phase particles in the low-matched joints can promote the crack growth.