Molecular dynamics simulation based on the newly developed embedded atom method has been performed to explore the microstructure of liquid Fe50Cu50 alloy. The results show that coordination numbers (CNs) of Fe-Fe and Cu-Cu for Fe50Cu50 melt gradually increase with relaxation time increasing, and they are 9.9 and 9.3 respectively as the liquid is in an equilibrium state; while the CN of heterogeneous atomic pairs Fe-Cu gradually decreases, and it is about 4.6. The correlation length (CL) extracted from Bhatia-Thornton (B-T) structure factor increases with relaxation time increasing. Both CN and CL indicate that the Fe50Cu50 melt exhibits liquid-liquid (L-L) phase separation. The interconnected type of structure can be observed in the Fe50Cu50 melt at the early stage, then the heterogeneous atomic pairs separate gradually with time going by, the Fe-rich and Cu-rich structure are formed, which shows the characteristics of spinodal decomposition. By comparison, the atom snapshot of Fe75Cu25 melt is also visualized in the paper, and the finding indicates that the smaller number difference between Fe atom and Cu atom may lead to the stronger L-L phase separation, as a result of shorter time to reach stable layer-like structure. Our studies mentioned above characterize L-L phase separation of metallic liquid on the atomic scale.