The influences of Bi addition on morphological modification of $Mg_2Si$ phase and mechanical properties were investigated in Mg-7%Al-0.5%Si casting alloy. It was found that the addition of 0.3%Bi changed the $Mg_2Si$ morphology from coarse Chinese script type to polygonal type, and significantly decreased the size to ~5 ${mu}m$ or less with the increase of number density. The modification of $Mg_2Si$ phase by the addition of Bi resulted in the improvement of tensile properties of the Mg-Al-Si alloy at RT and $175^{circ}C$.