The solidification process of Mg15Al binary magnesium alloy was controlled by the rotating magnetic field (RMF) to obtain even and small equiaxed grains, which make the followed equal channel-angular pressing proceed. The effect of various-strength rotating magnetic field on microstructure and the solute distribution of the Mg15Al binary magnesium alloy was investigated by OM, SEM and EDS. The results show that the distribution of the temperature field and solute field can be obviously improved by the severe fluid flow from the interaction between the RMF and the liquid metal, so the primary phase α-Mg in the as-cast Mg15Al binary alloy can be refined and the solid solution of aluminium in the primary phase α-Mg can be increased by RMF. But the eutectic microstructure has no change by RMF since no effective flow is formed in the end of solidification. The grain refinement increases gradually with the increase of magnetizing voltage. When the magnetizing voltage reaches 60 V, the finest grain and the highest solid solution of aluminium in the primary phase α-Mg can be obtained. But for the magnetizing voltage higher than 60 V, the grain size will decrease partially as the magnetic field-induced heat weakens the secondary cooling.