Abstract: The phase field method coupled with lattice Boltzmann algorithm were used to simulate the evolution of dendrite morphology of Mg-RE alloy under natural convection and forced convection. The differences of dendrite morphology of equiaxed, columnar and columnar-equiaxed transformation of Mg-RE alloy under natural convection and forced convection were discussed, and their concentration changes under flow field were analyzed. The results show that natural convection can form vortex at the tip of dendrite and affect the growth rate of dendrite. The effect of forced convection changes the solute distribution in the upstream and downstream of dendrites, and then affects the morphology of dendrites. The effect of forced convection makes the dendrite near the left boundary of the simulation region be inhibited to varying degrees.