The practical phase constituent diagram has been used to determine the composition of a low alloying τ-type Mg-Zn-Al alloy, which has a nominal element content of 7wt%Zn and 3wt%Al (ZA73), as a basis for further improvement of τ-type alloy through appropriate thermal processing and micro-alloying. The microstructure and solidification characteristics of the alloy have been experimentally examined using optical microscopy, scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry analysis. The results confirm that the as-microstructure of ZA73 alloy consists of globular equiaxed dendrite with τ phase as the only secondary phase evenly distributed in interdendritic spacing. Solidification sequence has been proposed with the help of DSC analysis and Mg-Zn-Al ternary liquidus projection phase diagram. Consistent with previous observed results for τ-type Mg-Zn-Al alloy, due to the decrease of element contents, ZA73 alloy has higher liquidus temperature (~627°C) and wider solidification range (~283°C), while at the mean time lower starting temperature of the second phase transformation (~354°C), compared to higher element containing ZA104 alloy. The phase constituent diagram has been shown to be a practical and effective tool for predicting the as-cast microstructural constituent of high zinc Mg-Zn-Al alloys under normal permanent mould solidification condition.