Abstract: An accurate three-dimensional core model was established for the melting and collapse problem of HPR1000 reactor core. The time stepping method was used to solve the movement and heat transfer equation of melting material, and the transient position and temperature of melting material in the core were determined to simulate the core heating and melting process. The simulation results show that the melting process begins to occur about 2 400 s after the shutdown, and the melting material falls in the active region of the reactor core and occurs multiple phase transition. After 4 900 s, the melting material forms a stable molten pool about 1.5 m high at the core bottom. Because of the heat transfer between the peripheral assemblies and the peripheral device, the peripheral assemblies will not melt. The analysis model of core melting material movement and heat transfer and the related calculation results established in this paper can provide technical reference for accident mitigation and treatment.