A simulation study is performed on the effects of six different cooling rates on microstructural evolution during solidification process of liquid Ca50Zn50 alloy with larger atomic size difference by using the molecular dynamics method. The pair distribution function, Honeycutt-Andersen (HA) bond-type index method, cluster-type index method (CTIM-2) and three-dimensional visualization method are adopted to deeply analyze the microstructural evolution. The results show that there is a critical cooling rate (in a range of 11012 and 51011 K/s) for forming amorphous or crystal structure. When the cooling rate, such as 11014 K/s, 11013 K/s, 11012 K/s and 51011 K/s, is above the critical cooling rate, the amorphous structures are formed mainly to be the 1551, 1541 and 1431 bond-types or the icosahedron basic clustr (12 0 12 0 0 0); while the cooling rate is under the critical cooling rate, such as at 11012 K/s, the partial crystal structures are formed mainly to be the 1441 and 1661 bond-types or the bcc clusters (14 6 0 8 0 0) (containing part of hcp (12 0 0 0 6 6) and fcc (12 0 0 0 12 0) basic crystal clusters) in the system. In the cooling rate range of forming amorphous structure, the first peak of the pair distribution function g(r) is split obviously into three secondary peaks corresponding to the nearest neighbor as Zn-Zn, Ca-Zn and Ca-Ca, respectively, and with the decrease of cooling rate, the secondary peak formed by the like atoms is inereased and the secondary peak formed by unlike atoms is reduced. With the decrease of cooling rate, the Zn atoms can be easily segregated to form the larger clusters; the lower the cooling rate, the bigger the number of basic icosahedrons formed in the system, and the amorphous system is more stable. In the cooling rate range of forming crystal structure, a great number of Zn atoms are segregated to form the bulk bcc crystal structures and part of Ca atoms are segregated to form some hcp and fcc crystal clusters.