A molecular dynamics simulation study has been performed for a large system consisting of 100000 liquid metal Al atoms to investigate the formation and magic number characteristics of the cluster configurations formed during the rapid solidification processes. The cluster-type index method (CTIM) has been adopted to describe various types of cluster configurations. The results indicate that the icosahedral clusters (12 0 12 0) and their combinations play the most important role in the microstructure transitions during solidification processes; for the cluster configurations of different levels formed by various combinations with differing numbers of basic clusters, their size distributions possess obvious magic number sequence which is in turn as 13(13), 19(21), 26-28(27), 32-33(32), 39-40, 43-44, 48..., (those in bracket are the corresponding value in liquid state); the magic numbers correspond to the peak value positions of the cluster numbers for corresponding level formed with various combinations by 1, 2, 3, 4, 5, 6, 7, ...basic clusters, respectively. This magic number sequence is in good agreement with the experimental results obtained by Harris et al. At the same time, this simulation study also gives a scientific and reasonable explanation to these experimental results.