The grain refinement mechanism of hypoeutectic Al-7%Si alloy under low voltage alternating current pulse (LACP) has been investigated in this work. In which LACP generated by the homemade low voltage modulation pulse generator is imposed in different solidification stages of the alloy and the wire mesh tubes of different diameters which have the effect of limiting the melt convection is embedded in sand mould. The experimental results show that the grains of casting alloy are evenly refined under LACP. The grain refinement will not appear, when LACP is imposed in the stages which are the alloy melt temperature is above 620 ℃ and late stages of crystal growth of primary phase. The grain refinement mainly occurs in nucleation stage and early growth stage of primary phase. The grains of inside and outside of wire mesh tube are refined together under LACP. But the grains of outside of wire mesh tube are much finer. The solidification microstructure of outside of wire mesh tube changes from large dendritic crystal to rose-shape crystal, nevertheless, it is still large dendritic crystal inside of wire mesh tube. Inocu lation effect and Joule heat effect of LACP have little effect on grain refinement of Al-7%Si alloy. The main reasons of grain refinement are the embryos fell off from chilling walls under LACP and the nucleation kinetics of the alloy was changed by LACP, which cause the nucleation rate of alloy increased. In addition, the α-dendrites became fragmentation under the forced melt flow which was induced by electromagnetic force can also lead to the grain refinement during the early growth stage of primary phase.