Pulse magneto-oscillation(PMO)was developed as a novel technique to refine the solidification structure of pure aluminium. Its grain refining mechanism was proposed. The PMO refinement mechanism is that the nucleus falls off from the mould wall and drifts into the melt under the action of PMO. The solidification structure of Al melt depends on the linear electric current density,and also the discharge and oscillation frequencies. The radial pressure of PIVIO sound wave is the major factor that contributes to the migration of nucleus into the melt.