Abstract: Effect of humidity on crack sensitivity of aluminum alloy weld joint and its mechanism were studied by EDS, SEM and XRD. Results showed that under the 40 % ~ 80% relative humidity conditions, solidfication cracks and liquation cracks on the surface of modified herringbone specimens were observed. With the increase of humidity, the crack sensitivity of the joint increased gradually, in which the sensitivity of the solidfication crack decreased, and the sensitivity of liquation crack increased gradually; As humidity increasing, the grain size of the aluminum alloy weld seam was gradually refined, the crack propagation resistance increased, which led to the reduction of the sensitivity of the solidfication crack. At the same time, as humidity increasing, the arc temperature decreased, and the welding input decreased. It was difficult for liquid metal to form continuous network film structures at grain boundaries, which also reduced the sensitivity of solidfication crack.