Coupled with the results of a 2D heat transfer model, a 3D electromagnetic stirring round billet model is developed, which is considered for the difference in the conductivity of solidified shell and molten steel. The electromagnetic field distribution features of the billet and the effect of round billet sizes on the electromagnetic field are investigated. It is found that as the solidified shell conductivity of the Φ600 mm round billet increases from 7.14 × 105S·m−1to 1.0 × 106S·m−1, the magnetic induction intensity decreases and the maximum value of electromagnetic force drops from 7976.26 N·m−3to 5745.32 N·m−3. The magnetic induction intensity on the center axis of the stirrer rarely changes in the range of Φ100–Φ200 mm. With the increase in the round billet from Φ300 mm to Φ600 mm, the magnetic induction intensity and the electromagnetic force on the center axis of the stirrer decrease slowly and then significantly. In the range of 2–8 Hz, as the current strength reaches its maximum, the electromagnetic force can be increased by increasing the current frequency for round billets of Φ100–Φ500 mm, while there is an optimal current frequency for round billets larger than Φ600 mm.