Abstract: A mathematical model was developed by Eulerian method during the solidification process of the hypermonotectic Al-Bi alloy in immiscibility region. A numerical simulation was done for the moving behavior of the liquid-phase decomposition of the alloy and its liquid-liquid separation, thus discussing the effects of two-phase temperature field, moving velocity, concentration distribution and other physical parameters on the microstructural evolution during solidification. The results showed that the undercooling in locally prenucleated region is the main influencing factor on the formation of local big-size droplets. Taking no account of gravity, the Marangoni force drives the second-phase droplets to migrate to the casting center, thus resulting in the ununiform distribution of the alloy. The ununiform distribution of concentration is the precondition under which the macrosegregation forms. Eventually, the directional motion results in the two-phase macrosegregation.