The tensile properties and microstructure of LF6 aluminum alloy welded joint under conditions of vacuum and thermal cycling have been studied by means of precise multifunction material testing system, constraint thermal cycling system, transmission electron microscope and scanning electron microscope so as to investigate the influence of thermal cycling on the tensile behavior of LF6 alloy welded joint. The results show that the vacuum and thermal cycling could increase the strength and elongation of the LF6 alloy welded joint. Both the strength and elongation are increased after thermal cycling, and the improvement that is influenced by numbers of thermal cycling is obviously for the welding joint. At the initial stage of thermal cycling, the strength and elongation of the LF6 alloy welded joint increase fast as a result of the generation of dislocation. The strength and elongation of the LF6 alloy welded joint reach their maximum values after cycling for 70 cycles. After that, the strength and elongation of the LF6 alloy welded joint decrease with proceeding of thermal cycling, which is resulted from the decrease of dislocation density within grain and formation of dislocation walls at the grain boundaries.