The rapid solidification of Al50Mg50 alloy melt has been simulated by using molecular dynamics method, and the evolution mechanism of microstructures at different levels during the rapid solidification process have been deeply studied with the pair distribution function, the bond-type index method and the cluster-type index method. It is demonstrated that the simulated Faber-Ziman partial structure factors are in good agreement with the experimental results. The Al50Mg50 alloy melt possesses heredity. The Al50Mg50 amorphous structure is formed during the rapid solidification process, and the icosahedron short-rang-order structure plays a critical role in the formation of amorphous structure. The atomic cluster analyses based on the cluster-type index method shows that the microstructures of Al50Mg50 amorphous alloy is built from some different types of short-rang-order structures, and the icosahedron is the main short-rang-order structure. There are various cluster structures with different sizes in the short-rang-order regions. But it can not be described with the Bernal model.