In this paper, the micro-damage mechanisms of 5A06 Al alloy weld joints have been studied under the condition of constant load and cyclic thermal load. The mechanical performance variation of the base material and its weld joint are analyzed and compared. Microstructure analysis reveals that the main damage mechanism in weld joints is the interior voids nucleation and growth. The voids distribution and evolution govern the damage process. Test results also show most fractures occur at HAZ near the welding fusion line. The development of these voids results in the performance deterioration of the weld joints under thermal cycling condition.