Heat transfer of molten slag under high temperature is significant for industrial processes. Unlike other phase change materials, the phase change temperature of molten slag is always not a constant during cooling process. In this case, the basis of judging different phases could not be accurately obtained, which brings difficulties for the calculation of radiation-solidification coupled heat transfer. In present work, the coupled conductive and radiative heat transfer of molten slag during solidification is investigated by the zonal method with fixed grid and moving area. In order to determine the liquid fraction of molten slag during phase change, the enthalpy method is improved by using crystallization kinetics. In addition, ray tracing method based traversal algorithm is developed to analyze the multiple reflection among three layers. The results indicate that the diffuse radiation in multi-layer could be described by tracking the path of reflections. However, the ray tracing method is time-consuming for computation. Compared with the ray tracing method, it is more efficient to calculate the radiative heat flux by solving a system of linear equations established according to the energy balance on each surface element. In initial stage of solidification, radiation is the main way of heat transfer. After solid slag is formed, heat transfer is dominated by conduction, and the proportion of the radiative heat flux in the total heat flux is lower than 30%. (C) 2017 Elsevier Ltd. All rights reserved.