Abstract: On the base of theoretical analysis on the medium-frequency heating of steel tubes, an electromagnetic-thermal coupling model of steel tubes was constructed with the Maxwell equations and the heat conduction equation. The electromagnetic and thermal fields of steel tubes were numerically simulated with Ansys software in consideration of the effect of the change in physical properties of materials with temperature on the heating process. The calculation results agree with the measured ones, for the surface temperature error of the heated workpieces was evaluated at 5.19%. The concept of induction heating penetration, which is used to divide the induction heating area and the heat transfer area during induction heating, was introduced. According to the simulation results, the induction heating penetration and the temperature distribution under different heating conditions were discussed. It is proved that the twin-coil heating mode is reasonable in distributing the temperature of the heated workpieees and assigning the heating efficiency and frequency.