High quality magnesium oxide (MgO) single crystals have been grown by an arc-fusion method. Due to the hostile environment for observing the process occurring in the inner zone of the furnace, direct measurements on the arcs by conventional diagnostic method are practically impossible. Moreover, it has been proved that the control of the temperature field of the whole system is crucial to grow high-purityMgO single crystals with large and super-large sizes. However, very few studies have been carried out to investigate the correlation between the thermal behavior and MgO single crystal growth in the furnace. In present study, a transient 3D finite element model has been developed to depict melt/crystal interface and to find out the determining factors that affect the crystal growth rate and the growth direction. It is shown that the model gave a good description of the bath shape which agrees well with that of the solidified MgO in experiments. By the transient analysis, the trend and time delay of the temperature variation on the shell were also predicted.