Abstract: 4 mm thick 1561 aluminum alloy was friction stir welded by the constant pressure controlling mode, and the microstructures of the FSW joint were studied. The results showed that the grain size first increased and then gradually decreased from the base material to the weld center, and the low-angle boundary proportion and the dislocation density continued to decrease. In HAZ, the grains grew under the action of the welding thermal cycle. The grains in TMAZ were composed of elongated grains and fine equiaxed crystals, and partial dynamic recrystallization occurred in this zone. The grains in SZ were fine equiaxed, indicating that complete dynamic recrystallization occurred. Compared with the precipitates with smaller number and larger size in the base metal zone, there were a large number of Al₆Mn phases with smaller size in the SZ, indicating that the precipitates in this zone dissolved first and then precipitated. At the same time, there were obvious substructures such as dislocation walls and subgrain boundaries in the SZ. This indicated that continuous dynamic recrystallization occurred. The Al₆Mn phase mainly distributed on the dislocations in the SZ, and this could pin the dislocations and hinder the growth of recrystallized grains.