The 4043 aluminum alloy thin–wall was manufactured by arc additive manufacturing (AM) process with forced water cooling constraint, and the macro–appearance and microstructure of AM specimens were studied. The results revealed that the verticality of the sample and the surface forming precision were improved. The average grain size decreased from 25μm to 12μm when the water cooling constraint was used, and the grain size was obviously refined. When the water cooling constraint device was applied, the 4043 aluminum alloy lateral wall was directly in contact with the restraint device, and the heat quickly dissipated. Therefore, the columnar grain grew in a certain angle. The hardness decreased from the bottom to the top of the AM specimen, and the average value of hardness increased from 48HV without restriction to 58HV. The horizontal grain distribution was cellular grain on both sides, central region was columnar grain, and the hardness of central region was lower than that of both sides.