A tracing simulation has been performed for the solidification of liquid Pb at six different cooling rates by molecular dynamics method. The pair distribution function g(r) curves, the bond-type index method of Honeycutt-Andersen(HA), the cluster-type index method and average coordination numbers have been used to analyze the evolution of micro-cluster structures during solidification processes. The results show that there is a critical cooling rate (in the range of 1×1013 and 5×1012 K·s-1) for forming amorphous or crystal structure. When the cooling rate is higher than the critical cooling rate, amorphous structures are formed mainly with the 1551,1541 and 1431 bond-types. When the cooling rate is lower than the critical cooling rate, the crystal structures mainly with the 1441 and 1661 bond-types or the bcc basic cluster (14 6 0 8 0 0) are firstly formed, and keep stable for a period of time, and then rapidly transform to the partial crystalline structure mainly with the 1421 and 1422 bond-types, or the fcc basic cluster (12 0 0 0 12 0) and the hcp basics cluster (12 0 0 0 6 6) coexisting in a certain proportion . At the same time, it has been found that there are obvious effects of the cooling rate on the relative proportion of the fcc basic clusters to the hcp basic clusters, the smaller the cooling rate is, the bigger relative proportion of the fcc basic cluster, and the system tends to form highly perfect fcc crystal structure.