To systematically investigate the effect of Sr addition on the second phases in the as-cast Mg-8Zn-4Al magnesium alloy and provide the theoretical basis for the design of the Mg-Zn-Al series alloys containing Sr element, the type and formation mechanism of second phases in the ZA84 as-cast magnesium alloys with low Sr mass fractions (0.1%, 0.3%, and 1.0%) were 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 ZA84 as-cast alloy without Sr addition, besides the quasicrystal phase (Q), an extra Mg₃₂(Al, Zn)₄₉ phase with a small amount is found to exist in the alloy. With Sr addition, there are Al4Sr and Mg₃₂(Al, Zn)₄₉ phases in the as-cast alloy; the Al4Sr phase increases and Mg₃₂(Al, Zn)₄₉ phase decreases with increase of Sr content. In the Mg₃₂(Al,Zn)₄₉ phases of the as-cast Mg-8Zn-4Al-(0.1-1)Sr magnesium alloys, Sr mainly exists in two forms, i.e. indissolved and dissolved Sr atoms; in the Mg₃₂(Al,Zn)₄₉ phase where Sr atoms are not dissolved, the Zn concentration is relatively high and Al concentration is low. The results provide the necessary theoretical support for the design of reinforced and heat-resistant Mg-8Zn-4Al alloy.