In view of the characteristic of VGF (Vertical Gradient Freeze) technique, the radiation heat transfer model of the VGF furnace is established, and detailed simulations of the radiation heat transfer in the VGF system using a finite difference method under the varying temperature condition is presented. The equilibrium temperature profiles of the ampoule are gained by the numerical method and the comparison with tailored temperature of the furnace wall shows a good agreement with the simulations. The results drawn in the paper can be used as thermal boundary conditions in the simulation of growth process, and give the directions of practical temperature control.