In this study, we explored the effect of an active afterheater on the growth of gallium oxide single crystals using the EFG method. We analyzed the temperature distribution of the crystal under the growing process through multiphysics simulations of the models with and without an active afterheater and investigated the morphology of crystals by applying each model to real experimental growths. The afterheater is a component in the growing furnace that activates radiant heat transfer, and its performance depends on its location, size, material, and shape. The simulation results showed that the afterheater applied in this study was found to be effective in obtaining good temperature distribution in the reactor. Through experimental crystal growth corresponding to the simulation approaches, it was confirmed that an appropriate afterheater reduces thermal stress at the growth front and provides a thermal annealing effect on the post-grown crystals during the growing process to improve crystal quality.