The samples with different strontium contents of AE42 magnesium alloy were prepared. The metalloscope and electronic microscope and XRD were used to characterize the microstructure and composition of the samples. The elevated temperature mechanical properties of the samples were tested and the strontium phases in the AE42 and their influence to the mechanical properties analyzed. The results showed that the main strontium phase in the AE42 was Al4Sr. Al4Sr pinned the crystal boundary of magnesium matrix and hindered dislocation climbing and sliding through the crystal boundary effectively. As a result, the crystal axial range was reduced and the crystal grain refined, and hence the creep resistance increased.