Based on the KKS model which couples the temperature field, concentration field, flow field and orientation field, the phase-field model for the magnesium alloys with hcp structure was established to simulate both of the single and the multi-grain dendritic growths of magnesium alloys during solidification under forced flow. The results show that the growths of three directions on the upstream side are much faster than those on the downstream side when the forced flow velocity is 0.02 m/s. At the same time, the primary dendrite on the upstream side is the longest, the two dendrites with level direction of 60o angle on the upstream side are second, the two dendrites with level direction of 60o angle on the downstream side are more shorter, and the primary dendrite on the downstream side is the shortest. Under the case of the multi-grain, the grains grown face to face influence mutually and grow competitively. The dendrite arms of different dendrite collide each other and are mutually inhibited, ultimately an unsymmetric dendrite morphology is formed. Based on the continuous nucleation model of the multi-grain dendritic growths of magnesium alloys, the simulation results show that there is a high consistency of the dendritic morphologies between simulation and experiment, verifying the validity of the KKS phase-field model