In order to systematically investigate the effect of Sr addition on the second phases in the as-cast AZ31 magnesium alloy and provide the theoretical basis for the design of the Mg-Al-Zn series alloys containing Sr element，the type and formation theory of second phases in the AZ31 as-cast magnesium alloys with low Sr contents(mass friction of 0.1，0.3，and 0.5) are investigated and analyzed by using scanning electron microscope(SEM)，energy dispersive spectrometer(EDS)，differential scanning calorimetry(DSC)，and X-ray diffractometer(XRD). The results indicate that for the AZ31 as-cast alloy without Sr addition，besides the Mg17Al12 phase，the extra Mg21(Zn，Al)17 phase with small amount is found to exist in the alloy. In addition，the difference both in the volume fraction and the type for the second phases in the AZ31 as-cast alloys with different Sr contents is very obvious. The alloys with the addition of 0.1 and 0.3 Sr(mass friction) mainly consists of the α-Mg，Mg17Al12，Mg21(Zn，Al)17 and Al4Sr phases. However，only the α-Mg，Mg21(Zn，Al)17 and Al4Sr phases are observed for the alloy with the addition of 0.5 Sr(mass friction). Otherwise，the volume fraction of the Al4Sr phase with lamella-like sharps increases with the Sr contents increasing among the 0.3-0.5(mass friction) range.