Molecular dynamics simulation is performed to characterize the concentration fluctuation of FeCu melts during the liquid-liiquid phase separation process,which undergoes the following stages:the formation of interconnected structure and its coarsening,migration and coagulation of droplets driven by the decreasing of potential energy.The up-hill diffusion happens at the early relaxation period in which Cu atoms in Fe-rich region are forced to move toward Cu-rich region by spinodal decomposition with 90％ Cu content in Cu-rich region and 95％ Fe content in Fe-rich region at temperature of 1500K.The higher diffusion rate of homogeneous atom can be observed at lower temperature,which is attributed to the larger potential energy difference between Cu-rich region and Fe-rich region.It also exhibits energy heterogeneity in the separated liquid.The domain size decreases sharply during the aggregation and coarsening of droplets,after that it keeps unchanged until the coagulation of droplets begins.The studies characterize concentration and energy heterogeneity of phase-separated Bquid on the atomic scale.