In this work, the Al100-x(Bi45Sn55)(x) (x=5, 15, 25 and 35) alloys with varied elemental compositions were prepared to understand the correlations of the elemental composition with the solidification behaviors and structures of the alloys. The results showed that the S-fh, S-mc, S2(Sn-Bi-rich, Al-rich) and (Sn-Bi-rich, Al-rich, Sn-Bi-rich) structures were formed successively with the decreasing aluminum content, due to the increased aggregation degree of droplets. The increased aggregation resulted from (1) the increased width of the miscibility gap (from 0 to 146K); (2) the increased volumetric fraction of the minority phase particles (from 8.91 to 49.98%); (3) the increased time of the L-LPS (from 0 to 0.46s); (4) the decreased solidification rate (from 10.2 to 4.6mms(-1)). The exchange of core and shell occurred in as-cast Al-Bi-Sn immiscible alloys. The quantitative relationship between the composition and the collision probability of droplets was established to reveal the inner reasons for structural evolution. In addition, the hot-spot effect of the lower melting point droplets was responsible for the coarser monotectic structure around the lower melting point particles. The results from this work are a useful reference for regulating structural configuration of immiscible alloys via manipulating the composition.