Semi-solid Al-6Si-2Mg alloy billets with fine spherical grains were prepared by self-developed vibrating wave-like sloping plate device,and effects of different reheating processes on microstructure of the billets were observed. The results reveal that solute diffusion is accelerated,and liquid region near grain boundary tends to be continuous and liquid fraction rises with increasing in holding temperature at a given holding time. Due to the role of interface free energy,average grain size exhibits firstly increase and then decrease,meanwhile,average grain roundness and density show gradually decrease. At a given holding temperature,with extending in holding time,eutectic liquid phase is increasingly raised to be formed a continuous liquid network to separate the grain. To decrease interface free energy,grain size exhibits firstly increase and then decrease,while grain roundness is decreased with raising in liquid fraction. Desirable semi-solid slurry with spherical grain suspending on liquid phase can be obtained with holing at 610 ℃ for 60 min.