Abstract: In this paper, a solidification model of magnesium alloy based on the Eulerian multiphase flow technique and cellular automata method is proposed to investigate the evolutions of aluminum concentration and solidification microstructure during the directional solidification of magnesium alloy with 9% (mass fraction) aluminum concentration under three types of boundary conditions, i.e., no flow, forced flow in the x-direction, and forced flow in the y-direction. The numerical simulations reveal that the dendrites of magnesium alloy grow at an angle of 60° to each other for the no-flow condition. In addition, secondary dendrites are also found in the late period of solidification, which grow at an angle of 60° to the primary dendrites. Both characteristics of primary dendrites and secondary dendrites demonstrate that the simulated solidification microstructure has a characteristic of crystal solidification with the hexagonal closed-packed structure, which confirms the reliability of the model. For the condition of forced flow in the x-direction, the main difference between this case and the no-flow condition is that the dendrites grow faster along the direction of the melt flow. Moreover, the characteristics of well-developed secondary dendrites are also found in the late solidification period for this condition. The main reason for current solidification phenomena is that the rejected solute in the vicinity of liquid–solid interface is transported along the melt flow during the solidification and accumulates in the rear of the dendrites. The decrease in aluminum concentration due to the melt flow in the area of dendritic tips will increase the supercooling, which finally advances the dendrite growth. Conversely, for the condition of forced flow in the y-direction, asymmetric growths of dendrites appear, and the preferred orientation for some dendrites has deflected about 3° compared with the no-flow condition. The numerical simulations indicate that the rejected solute is transported along the melt flow from one side of the dendrite to another side due to the forced flow in the y-direction, which results in the asymmetric distribution of aluminum concentration and the forming asymmetric characteristics of dendrites. Meanwhile, \boldsymbolu_\textl\nabla c_1 represents the formation mechanism of the asymmetric morphology of dendrites. The results show that it promotes solidification when \boldsymbolu_\textl\nabla c_1 > 0 and suppresses the dendrite growth when it is negative. The \boldsymbolu_\textl\nabla c_1 in the vicinity of the liquid–solid interface is seen as asymmetric distribution due to the forced flow in the y-direction during the solidification, which finally results in the deflection of the dendrite growth.