Abstract: To precisely understand the accident process of reactor core melt in In-vessel retention（IVR） condition, 3 dimensional transient thermal conduction analysis with moving boundary is performed on quarter reactor core model. The decline of decay power and water level in reactor pressure vessel（RPV）, and the radial distribution of assemblies of different material is considered. Convective heat transfer on rod surface and coolant interface in computed with empirical correlation of natural convection of saturated steam vapor/water. Radiation heat transfer with 16 neighboring rod is considered. Also, the ablation caused by continuously accumulation of molten corium on lower support plate（LSP） is simulated. The impingement heat transfer of the falling corium and the molten pool formed in LSP ablation cavity is taken into account. The simulation gives the ablation process on the surface of LSP as well as temperature history and molten proportion of the reactor core, which shows agreement with reference. Simulation shows: the melt process of reactor core accelerated in the accident process of 2600 s, when coolant in RPV dry up 65% of the core mass has molten at 8000 second. LSP is completely penetrated in 6000 s, the ablation of LSP is mainly focused on an annular region of radius 700 mm.