The microstructure evolution and tensile mechanical properties of the isothermal die forged AZ80-Ag casing with large size under different heat treatment conditions were investigated. The microstructure was characterized by scanning electron microscope and electron backscatter diffraction techniques, and the tensile mechanical properties were tested at room temperature, 120 and 150 °C, respectively. The results show that the area fraction of discontinuous precipitates with stronger strengthening effect is higher in T5 sample than that in T6 sample, therefore T5 sample exhibits a more pronounced precipitation hardening and the highest tensile strength at room temperature. However, as the temperature increases, the tensile strength of T5 sample decreases rapidly and is lower than that of T6 sample or even the as-forged samples at 150 °C. The reason is that discontinuous precipitates are less resistant to high temperature softening than continuous precipitates.