For the core-shell structured immiscible alloys within the immiscible gap, whether the shell is composed of a phase with lower or higher melting point, there is no exact answer yet. Based on the Cu-Sn-Pb calculated phase diagram given by reference, the phase separation of 14Sn-73.1Cu-12.9Pb, 14Sn-64.5Cu-21.5Pb, Cu-65Pb and Cu-75Pb alloys have been experimentally investigated by casting method. An anomalous core-shell structure with a CuSn-rich shell and a Pb-rich core formed in 14Sn-64.5Cu-21.5Pb alloy, which differed from the viewpoint that a lower melting point phase generally formed the shell as Cu-75Pb alloy showed. This anomalous phenomenon may be explained by that the liquid separation reaction was suppressed due to undercooling generated during casting process and the monotectic reaction took place before the liquid separation reaction. This work may provide a method to modify the structure of immiscible alloys through controlling solidification path.