A transient analysis mathematic model was established via ANSYS. The variation laws of magnetic induction density, induced eddy current and electromagnetic force in molten steel changing with the high frequency sine exciting current were investigated. The results indicate that the magnetic induction density and induced eddy current in molten steel vary at the same frequency as exciting current. Moreover, the variations of magnetic induction density and induced eddy current are not in step with exciting current. This proves that the high frequency electromagnetic field in mold and steel is not a time-harmonic wave. The electromagnetic force varies at the double frequency of the exciting current. When the exciting current reaches the amplitude value, the electromagnetic force is close to the max value, whereas when the exciting current returns to zero, the electromagnetic force is not zero. At the same time, there is partial electromagnetic force reversing its direction. If the reversed electromagnetic force is expanded seriously, the fluctuation of liquid level is induced, so it must be controlled.