In this paper, the influences of the melt superheating temperature on the solidification structure and mechanical properties of AZ91D magnesium are studied by a home-made electrical resistance furnace. The results show that, with the increase of melt superheating temperature, the as-cast structure of AZ91D alloy changes from dendrite morphology into equiaxed grain morphology, the grain size gradually decreases, while the change of the grain size is not obviously when the melt superheating temperature exceeds 850℃. With the enhancement of melt superheating temperature, the tensile strength, offset yield strength and specific elongation increase firstly and then decreases. The mechanical properties of AZ91D reach the maximum. The analysis of DSC show that the increase the melt superheating temperature makes the beginning solidification temperature reduce, shrink the freezing range, decrease the critical nucleus radius, increase the degree of supercooling of the melt and enhance the heterogeneous nucleation rate, these are the primary reason of grain refinement in magnesium alloy. Increasing the superheating temperature will lead to the increase of the eutectic transformation temperature, hence the eutectic phase is coarse.