The influence of different ratios of La and Ag under the condition of T6 peak aging treatment and different thermal exposure were investigated. The microstructures and tensile properties of Weldalite049 aluminum alloy were analyzed by SEM, TEM and tensile properties testing. The aim is to investigate the method of substituting a small amount of rare earth La for higher price Ag in precondition of satisfying mechanical requirements. It is proved that the tensile strength of Weldalite 049 alloy with 0.05wt.% La instead of 0.2wt.% Ag is 615.5MPa after T6 treatment, which is higher than that of Weldalite 049 alloy 611.1MPa. The mechanical properties of Weldalite 049 alloy with 0.1wt.% La but without Ag remain at a high level under thermal exposure at 175 C/60h.