In order to simulate the transient phenomenon of retention in high temperature core melt of nuclear power plant, the mixture of 50kg ZrO2-Fe-Zr system was melted by electromagnetic induction in a cold crucible. The phase and element distribution of oxide layer, metal layer and oxide crust of solidified melt ingot were analyzed by means of XRD, SEM, EDS and so on. The results show that during the formation of the 50-kg-scale initial molten pool, the high temperature molten material is separated into a two-layer structure of metal layer and oxide layer due to gravity and incompatibility, and an oxidation crust is formed on and around the top of the molten pool; After that, due to the secondary feeding to form a three-layer molten pool with the top liquid metal, the top metal molten pool can gradually settle to the bottom of the molten pool through the oxide hard crust.