The research achievements on as-cast microstructure in Mg-Al-Zn alloy were summarized. Under permanent mould cast condition, there are four kinds of primary compounds with distinct crystallographic morphology, Mg17Al12(γ),Mg32(Al,Zn)49 (τ), MgZn (ε) and a ternary icosahedral quasi-crystalline compound (Q). Accordingly, Mg-Al-Zn alloy can be grouped into γ-, τ-, ε- and Q-type alloy by each characteristic compound. The volume fraction of γ-Mg17Al12 in commercial γ-type alloy increases with increasing Al content. MgZn and MgxAlyZnz ternary complex compounds emerge with the change of the element content Al and Zn and Zn/Al concentration ratio. A practical phase diagram showing microstructure constituent change with composition was proposed. The addition of micro-alloying elements Y and Sr results in not only obvious refinement of eutectic cluster but also eutectic morphological change from block to granule.