The microstructure evolution of magnesium alloy AZ91D solidified with different electric current pulses and cooling rates was investigated and a new method, Low-voltage Electric Current Pulses (LVECP), to produce semi-slurry magnesium alloy was developed in this paper. The experimental results showed that the electric current pulses during solidification changed morphology of dendrites and the equiaxed, non-dendritic grains formed. The size of the primary alpha-Mg particles in semi-solid AZ91D alloy and the sphericity of the particles decreased with increase of discharging the voltage and treating time of LVECP. The increase of the cooling rate during the solidification of AZ91D alloy with LVECP promoted the formation of finer a-Mg particles, but the value of the sphericity of the particles rised. The formation of equiaxed, nondendritic structure by LVECP might be attributed to the electric current pulses increase the nucleation rate, restrained growth of the dendrites, and made dendrite arms remelted during the solidification of AZ91D alloy.