Abstract: In this paper, a solidification model of Mg alloy based on Eulerian multiphase flow technique and cellular automata method is proposed, and investigates the microstructure evolution during the directional solidification of Mg-9wt%Al alloy under three boundary conditions. The results show that the dendrites of Mg alloy grow at an angle of 60° to each other when there is no flow, and the secondary dendrites appear at an angle of 60° to the primary dendrites in the solidification simulation, which proves the reliability of the model. In the case with the x-directional flow, the dendrites grow faster in the flow direction and appear well-developed secondary dendrites, which is mainly due to the solute from the front of the dendrites being transported to the rear of the dendrites by the flow. Meanwhile, the dendrite shows asymmetric growth in the case with the y-directional flow, in which the dendrite growth direction is deflected by about 3° for some dendrites. The analysis shows that the asymmetric distribution of the values of at the solid-liquid interface during the dendrite growth is the main reason for the deflection of the dendrite growth.