The thermal stability of Al-Cu-Mg-Ag alloys under-aged and peak-aged was analyzed by tensile testing and transmission electron microscopy (TEM). And the microstructural evolution and mechanical properties of Al-Cu-Mg-Ag alloy during thermal exposed at elevated temperature were also studied. The results show that the under-aged Al-Cu-Mg-Ag alloy possessed better thermal stability compared with the peak-aged alloy when exposed at 150 . The tensile strength of the peak-aged alloy decreased with prolonging the holding time. While that of the under-aged alloy increased and then decreased with a peak of 524 MPa after exposed for 20 h, which was 19 MPa higher than that of the peak-aged alloy. Meanwhile the precipitations were distributed dispersively and amount of fine phases were observed. The tensile strength of the under-aged alloy was 434 MPa after exposed for 1000 h at 150 . When exposed at 200 , the tensile strength of the under-aged sample decreased and the prolongation increased with prolonging the thermal exposure time. The tensile strength of the sample decreased dramatically when exposed at 250 and 300 and the strengthening phases coarsened dramatically and evolved to the equilibrium phases . The width of the precipitation free zone (PFZ) increased with increasing the thermal exposure temperature and the distribution of the precipitations on the boundaries became discontinuous.